Tough Puzzles & Brain Teasers
Puzzles on logic and mathematics that will make you scratch your head.
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Kunal Gokhe
Mechanical
05 Feb 2019

# Five Pirate Puzzle

5 pirates of different ages have a treasure of 100 gold coins.

On their ship, they decide to split the coins using this scheme:

The oldest pirate proposes how to share the coins, and ALL pirates (including the oldest) vote for or against it.

If 50% or more of the pirates vote for it, then the coins will be shared that way. Otherwise, the pirate proposing the scheme will be thrown overboard, and the process is repeated with the pirates that remain.

As pirates tend to be a bloodthirsty bunch, if a pirate would get the same number of coins if he voted for or against a proposal, he will vote against so that the pirate who proposed the plan will be thrown overboard.

Assuming that all 5 pirates are intelligent, rational, greedy, and do not wish to die, (and are rather good at math for pirates) what will happen?

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2mos ago

A bit involved.

If the oldest three get eliminated leaving the last two, the fourth can take it all giving nothing to the fifth, whose vote being only 50% will not count against the proposal. So the fifth will avoid voting that will lead to only the last two being left.

If three are left the third will make a proposal giving some small amount to the fifth, none to the fourth and keep the bulk to himself.

So the fourth will avoid getting into that scenario.

If the first gets eliminated leaving four, the second will make a proposal cutting out three and four and giving a token to five to ensure his vote. This will pass because the voting will be fifty-fifty.

Three and four will avoid this situation.

The best bet seems to be for number one to give something to four and five, cut out two and three, keeping the bulk for himself.

Kunal Gokhe
Mechanical
2mos ago

The oldest pirate will propose a 98 : 0 : 1 : 0 : 1 split, in other words the oldest pirate gets 98 coins, the middle pirate gets 1 coin and the youngest gets 1 coin.

Let us name the pirates (from oldest to youngest): Alex, Billy, Colin, Duncan and Eddie.

Working backwards:

2 Pirates: Duncan splits the coins 100 : 0 (giving himself all the gold). His vote (50%) is enough to ensure the deal.

3 Pirates: Colin splits the coins 99 : 0 : 1. Eddie will accept this deal (getting just 1 coin), because he knows that if he rejects the deal there will be only two pirates left, and he gets nothing.

4 Pirates: Billy splits the coins 99 : 0 : 1 : 0. By the same reasoning as before, Duncan will support this deal. Billy would not waste a spare coin on Colin, because Colin knows that if he rejects the proposal, he will pocket 99 coins once Billy is thrown overboard. Billy would also not give a coin to Eddie, because Eddie knows that if he rejects the proposal, he will receive a coin from Colin in the next round anyway.

5 Pirates: Alex splits the coins 98 : 0 : 1 : 0 : 1. By offering a gold coin to Colin (who would otherwise get nothing) he is assured of a deal.