Pirates' Gold Allocation Logic Puzzle
Topic 9:
Logic
Dr J Frost (jfrost@tiffin.kingston.sch.uk)
Last modified: 6
th
August 2013
ζ
Part 1: General Questions
Topic 9 – Logic
http://www.youtube.com/watch?v=JR4H76SCCzY
This… statement… is… false…
[Source: Junior Maths Olympiad]
How many of the statements in the box are true?
None of these statements are true.
Exactly one of these statements is true.
Exactly two of these statements are true.
All of these statements are true.
0
1
2
3
(Click your answer)
4
Suppose that 2 statements were true. Then the third statement is true. But
all the other statements would be false. This leads to a contradiction.
Question r)
On a ship there are
5 pirates
, all of whom are
ruthless
*,
greedy
and all of whom are perfect logicians.
They seize
100 gold coins
from Crisan Cove, and the captain, as the
most senior pirate, gets to decide how the treasure is divided up
amongst the pirates (including himself).
All pirates (including the captain) vote on whether they agree with
the allocation of gold. If
at least half agree
, then the deal goes
ahead, otherwise the captain is thrown overboard, and
the process
repeats
with the next most senior pirate as the new captain.
How should the captain allocate the gold to avoid and mutiny,
while keeping as much as possible?
Hint:
Think about the situation if there was just 2 pirates. Then reason about what would
happen with 3 pirates if each knew what would happen if the captain was thrown overboard.
Gradually extend to 5 pirates.
* A ruthless pirate would prefer to see the
captain die provided it doesn’t affect their
earnings.
Question r)
Solution:
Case A) There are 2 pirates
The first pirate can offer no gold, because by agreeing with his own deal, he constitutes
the 50% agreement needed.
Case B) There are 3 pirates
The third ranking pirate knows that he must accept whatever deal he is offered, because
if he disagrees and Pirate 1 is thrown overboard, he’ll now be Pirate 2, where we know
from the case above he’ll get nothing. Therefore, Pirate 1 can offer Pirate 3 just one piece
of gold, and Pirate 2 nothing. They’ll be two votes of agreement, which is over half.
Case C) There are 4 pirates
Pirate 3 knows that he’ll get nothing if Pirate 1 is thrown overboard, because of Case B
above. He would have to accept any offer. The captain could technically also give one
coin to Pirate 4, but Pirate 4, being ‘ruthless’, knows he’ll get one coin anyway when he
becomes Pirate 3.
Case D) There are 5 pirates
Now Pirate 1 must get two other pirates on his side to secure a majority. Pirate 5 must
accept any offer, because he would next be Pirate 4, where from Case C, he’ll get
nothing. Pirate 3 must accept any offer for the same reason.
Thus, Pirate 1 can give just one coin to Pirate 3, and one coin to Pirate 5.
C
B
A
4 men are kidnapped and buried in the sand, awaiting execution.
There’s four hats –
2 red
and
2 blue
. The men know that there’s two of each
colour, although obviously can’t see the hat they’re wearing.
All men are facing the wall. Hence C can see the hats of B and A. B can see the hat
of A. The guy on the other side of the wall can’t see anyone, nor can be seen.
The prisoners all escape execution
if any of them
can
work out the colour of their
own hat, and when one person does, they call out to announce it.
(This is just one possibility in terms
of how the hats are worn)
C
B
A
Solution:
B and A can either be wearing hats of the same colour or of different colours.
Case 1) Same colour
. Then person C knows he must be wearing a hat of the other colour, so
can call out.
Case 2) Different colours
. Then C can’t work out his colour. But person B, knowing C can’t
work out their own hat after C fails to say anything, knows he must be wearing a different hat
to A. So he just calls out the opposite of A’s colour.
This problem is an example of ‘
epistemic logic
’
, the logic of ‘knowledge’. It allows us to model
various people’s knowledge, and the effect of ‘announcements’ on people’s knowledge.
i.
Alice, Bob, Charlie and Dianne each make the following statements:
Alice: I am telling the truth.
Bob: Alice is telling the truth.
Charlie: Bob is telling the truth.
Dianne: Charlie is lying.
Only one of the 4 people is telling the truth. Which one? Explain your answer.
ii.
They now make the following statements:
Alice: Bob is lying.
Bob: Charlie is lying.
Charlie: I like beer.
Dianne: 2+2=4.
Now two of the four people are telling the truth. Which two? Explain your answer.
iii.
They are now joined by Egbert. They each make the following statements:
Alice: I like wine.
Bob: Charlie is lying.
Charlie: Alice is lying.
Dianne: Alice likes beer.
Egbert: Alice likes beer.
Now three of the five people are telling the truth. Which ones? Explain your answer.
i.
Alice, Bob, Charlie and Dianne each make the following statements:
Alice: I am telling the truth.
Bob: Alice is telling the truth.
Charlie: Bob is telling the truth.
Dianne: Charlie is lying.
Only one of the 4 people is telling the truth. Which one? Explain your answer.
If Bob is telling the truth, then so is Alice, contradicting the given fact
that only one person is telling the truth. So Bob is lying which means Alice
also is. Similarly Charlie is also lying. Hence Dianne is the one telling the
truth.
?
ii. They now make the following statements:
Alice: Bob is lying.
Bob: Charlie is lying.
Charlie: I like beer.
Dianne: 2+2=4.
Now two of the four people are telling the truth. Which two? Explain your answer.
Clearly Dianne is telling the truth. If Alice is telling the truth, then Bob is
lying and Charlie is telling the truth — but that leaves three telling the truth, a
contradiction. So Alice is lying, which means Bob is telling the truth and Charlie
is lying.
?
iii. They are now joined by Egbert. They each make the following statements:
Alice: I like wine.
Bob: Charlie is lying.
Charlie: Alice is lying.
Dianne: Alice likes beer.
Egbert: Alice likes beer.
Now three of the five people are telling the truth. Which ones? Explain your answer.
Now three are telling the truth. Either Dianne and Egbert are both telling the
truth, or both are lying. If they are both lying, the other three must be telling the
truth — but Charlie telling the truth means Alice is lying, a contradiction. So
Dianne and Egbert are telling the truth and one of the others. Either Alice and
Bob are each telling the truth and Charlie is lying, or vice versa. As only one of
these three is truthful it must be Charlie. So Charlie, Dianne, Egbert are telling
the truth.
?
ζ
Part 2: Boolean Algebra
Topic 9 – Logic
?
?
?
?
In algebra, letters/variables/constants represent values and we have a number of
operations which allows us to combine them.
“It is raining.”
“The ground is wet.”
“It is raining AND the ground is wet.”
Conjunction
Then what does the above represent?
?
?
?
?
?
“It is raining.”
“The ground is wet.”
“It is raining
OR
the ground is wet.”
Then what does the above represent?
?
?
?
?
?
¬a
“It is raining.”
“It is NOT raining.”
Then what does the above represent?
?
?
?
a: “It is raining.”
b: “The ground is wet.”
¬a
∨
b
(¬a
∧
b)
∨
(a
∧
¬b)
(a
∨
b)
∧
¬(a
∧
b)
¬(a
∨
b)
¬a
∧
¬b
Which of these statements are equivalent? (We could use a truth table to prove it).
These are equivalent. Their
equivalence is known as
De
Morgan’s Law
.
These are equivalent. They
both indicate “it’s raining
or wet, but not both”.
a
⟶
b
“It is raining.”
“The ground is wet.”
“
If
it is raining, the ground is wet.”
Then what does the above represent?
?
?
?
?
?
Perversely, this implies statements like the following are true:
“
If 2+2=5, I am the Pope
”. Because the condition is false, the statement is
vacuously true.
We only falsify an conditional statement if the condition is true, but the
implication is false.
a = “It is raining.”
b = “The ground is wet.”
Hypothesis 1
True
False
(Click to vote!)
a = “It is raining.”
b = “The ground is wet.”
Hypothesis 2
True
False
a = “It is raining.”
b = “The ground is wet.”
Hypothesis 3
True
False
Only the last one is true. This law in Latin is known as
“
modus tollens
”, which means “that way that denies by denying”.
There are 4 cards on the table. You know that all have a letter
on one side, and a number on the other.
A friend claims “If a card has a consonant on one
side, it has an even number on the other.”
I need to verify whether this claim is true or not.
What are the minimum number of cards I need to
turn over?
Answer: 2 cards
Note first that the implication is one way!
•
Turning ‘A’ over is unnecessary, because it
can’t falsify our statement regardless of
whether we see an odd or even number.
•
‘K’ is necessary, because if we saw say a 5
on the other side, the statement would be
false.
•
Similarly, ‘3’ is necessary, because if we
saw a ‘C’ say, the statement would be
false.
•
Turning the ‘2’ over is unnecessary,
because if it was a vowel, it wouldn’t
falsify our statement.
?
Explore a challenging logic puzzle involving 5 ruthless pirates dividing 100 gold coins amongst themselves to avoid mutiny. By applying strategic thinking and considering each pirate's incentives, discover the clever allocation strategy that ensures the captain's survival and maximizes their own earnings. Dive into the complexities of logical reasoning and decision-making in this intriguing scenario.
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Presentation Transcript
Topic 9: Logic Dr J Frost (jfrost@tiffin.kingston.sch.uk) Last modified: 6th August 2013
Topic 9 Logic Part 1: General Questions
A quick video This statement is false http://www.youtube.com/watch?v=JR4H76SCCzY
Starter Question How many of the statements in the box are true? [Source: Junior Maths Olympiad] None of these statements are true. Exactly one of these statements is true. Exactly two of these statements are true. All of these statements are true. (Click your answer) 1 0 2 3 4 Suppose that 2 statements were true. Then the third statement is true. But all the other statements would be false. This leads to a contradiction.
Question r) On a ship there are 5 pirates, all of whom are ruthless*, greedy and all of whom are perfect logicians. They seize 100 gold coins from Crisan Cove, and the captain, as the most senior pirate, gets to decide how the treasure is divided up amongst the pirates (including himself). All pirates (including the captain) vote on whether they agree with the allocation of gold. If at least half agree, then the deal goes ahead, otherwise the captain is thrown overboard, and the process repeats with the next most senior pirate as the new captain. How should the captain allocate the gold to avoid and mutiny, while keeping as much as possible? * A ruthless pirate would prefer to see the captain die provided it doesn t affect their earnings. Hint: Think about the situation if there was just 2 pirates. Then reason about what would happen with 3 pirates if each knew what would happen if the captain was thrown overboard. Gradually extend to 5 pirates.
Question r) Solution: Case A) There are 2 pirates The first pirate can offer no gold, because by agreeing with his own deal, he constitutes the 50% agreement needed. Case B) There are 3 pirates The third ranking pirate knows that he must accept whatever deal he is offered, because if he disagrees and Pirate 1 is thrown overboard, he ll now be Pirate 2, where we know from the case above he ll get nothing. Therefore, Pirate 1 can offer Pirate 3 just one piece of gold, and Pirate 2 nothing. They ll be two votes of agreement, which is over half. Case C) There are 4 pirates Pirate 3 knows that he ll get nothing if Pirate 1 is thrown overboard, because of Case B above. He would have to accept any offer. The captain could technically also give one coin to Pirate 4, but Pirate 4, being ruthless , knows he ll get one coin anyway when he becomes Pirate 3. Case D) There are 5 pirates Now Pirate 1 must get two other pirates on his side to secure a majority. Pirate 5 must accept any offer, because he would next be Pirate 4, where from Case C, he ll get nothing. Pirate 3 must accept any offer for the same reason. Thus, Pirate 1 can give just one coin to Pirate 3, and one coin to Pirate 5.
(This is just one possibility in terms of how the hats are worn) C B A 4 men are kidnapped and buried in the sand, awaiting execution. There s four hats 2 red and 2 blue. The men know that there s two of each colour, although obviously can t see the hat they re wearing. All men are facing the wall. Hence C can see the hats of B and A. B can see the hat of A. The guy on the other side of the wall can t see anyone, nor can be seen. The prisoners all escape execution if any of them canwork out the colour of their own hat, and when one person does, they call out to announce it.
C B A Solution: B and A can either be wearing hats of the same colour or of different colours. Case 1) Same colour. Then person C knows he must be wearing a hat of the other colour, so can call out. Case 2) Different colours. Then C can t work out his colour. But person B, knowing C can t work out their own hat after C fails to say anything, knows he must be wearing a different hat to A. So he just calls out the opposite of A s colour. This problem is an example of epistemic logic , the logic of knowledge . It allows us to model various people s knowledge, and the effect of announcements on people s knowledge.
MAT Question i. Alice, Bob, Charlie and Dianne each make the following statements: Alice: I am telling the truth. Bob: Alice is telling the truth. Charlie: Bob is telling the truth. Dianne: Charlie is lying. Only one of the 4 people is telling the truth. Which one? Explain your answer. They now make the following statements: Alice: Bob is lying. Bob: Charlie is lying. Charlie: I like beer. Dianne: 2+2=4. Now two of the four people are telling the truth. Which two? Explain your answer. iii. They are now joined by Egbert. They each make the following statements: Alice: I like wine. Bob: Charlie is lying. Charlie: Alice is lying. Dianne: Alice likes beer. Egbert: Alice likes beer. Now three of the five people are telling the truth. Which ones? Explain your answer. ii.
MAT Question i. Alice, Bob, Charlie and Dianne each make the following statements: Alice: I am telling the truth. Bob: Alice is telling the truth. Charlie: Bob is telling the truth. Dianne: Charlie is lying. Only one of the 4 people is telling the truth. Which one? Explain your answer. If Bob is telling the truth, then so is Alice, contradicting the given fact that only one person is telling the truth. So Bob is lying which means Alice also is. Similarly Charlie is also lying. Hence Dianne is the one telling the truth. ?
MAT Question ii. They now make the following statements: Alice: Bob is lying. Bob: Charlie is lying. Charlie: I like beer. Dianne: 2+2=4. Now two of the four people are telling the truth. Which two? Explain your answer. Clearly Dianne is telling the truth. If Alice is telling the truth, then Bob is lying and Charlie is telling the truth but that leaves three telling the truth, a contradiction. So Alice is lying, which means Bob is telling the truth and Charlie is lying. ?
MAT Question iii. They are now joined by Egbert. They each make the following statements: Alice: I like wine. Bob: Charlie is lying. Charlie: Alice is lying. Dianne: Alice likes beer. Egbert: Alice likes beer. Now three of the five people are telling the truth. Which ones? Explain your answer. Now three are telling the truth. Either Dianne and Egbert are both telling the truth, or both are lying. If they are both lying, the other three must be telling the truth but Charlie telling the truth means Alice is lying, a contradiction. So Dianne and Egbert are telling the truth and one of the others. Either Alice and Bob are each telling the truth and Charlie is lying, or vice versa. As only one of these three is truthful it must be Charlie. So Charlie, Dianne, Egbert are telling the truth. ?
Topic 9 Logic Part 2: Boolean Algebra
Boolean Algebra In algebra, letters/variables/constants represent values and we have a number of operations which allows us to combine them. Conventional Algebra Boolean Algebra Values for variables/constants Any number (-3.2, 6, 783, etc.) ? True (T) or False (F) ? + (Addition) - (Subtraction) (Multiplication) (Division) (Conjunction) (Disjunction) (Negation) ? ? Operators
Conjunction Suppose that the variables ? and ? represent the following true/false statements: It is raining. The ground is wet. ? ? Then what does the above represent? It is raining AND the ground is wet. ?
Conjunction a b a b F F F ? F T F ? T F F ? T T T ? This is known as a truth table . It gives us the value of the overall expression (here: ? ?) for all possible values for the variables.
Disjunction It is raining. The ground is wet. ? ? Then what does the above represent? It is raining OR the ground is wet. ?
Disjunction a b a b F F F ? F T T ? T F T ? T T T ?
Negation It is raining. a Then what does the above represent? It is NOT raining. ?
Negation a a F T ? F ? T
What do these mean? a: It is raining. b: The ground is wet. a b a b (a b) These are equivalent. Their equivalence is known as De Morgan s Law. ( a b) (a b) (a b) (a b) These are equivalent. They both indicate it s raining or wet, but not both . Which of these statements are equivalent? (We could use a truth table to prove it).
Implication It is raining. The ground is wet. a b Then what does the above represent? Ifit is raining, the ground is wet. ?
Implication a b a b F F T ? F T T ? T F F ? T T T ? Perversely, this implies statements like the following are true: If 2+2=5, I am the Pope . Because the condition is false, the statement is vacuously true. We only falsify an conditional statement if the condition is true, but the implication is false.
Implication a = It is raining. b = The ground is wet. Hypothesis 1 ? ? implies that ? ? (Click to vote!) True False
Implication a = It is raining. b = The ground is wet. Hypothesis 2 ? ? implies that ? ? True False
Implication a = It is raining. b = The ground is wet. Hypothesis 3 ? ? implies that ? ? True False Only the last one is true. This law in Latin is known as modus tollens , which means that way that denies by denying .
Example There are 4 cards on the table. You know that all have a letter on one side, and a number on the other. K 2 A 3 Answer: 2 cards Note first that the implication is one way! Turning A over is unnecessary, because it can t falsify our statement regardless of whether we see an odd or even number. K is necessary, because if we saw say a 5 on the other side, the statement would be false. Similarly, 3 is necessary, because if we saw a C say, the statement would be false. Turning the 2 over is unnecessary, because if it was a vowel, it wouldn t falsify our statement. A friend claims If a card has a consonant on one side, it has an even number on the other. I need to verify whether this claim is true or not. What are the minimum number of cards I need to turn over? ?
