Question

(Impossible) Twelve men are on a desert island. They all have identical weights except for one of them, who is either slightly lighter or slightly heavier than the others.

The only other thing on the island is a seesaw. There are no scales or means to measure weight otherwise. Can you determine which man has the different weight? You only get to use the seesaw three times.

Answer 1

Yes, it is possible to determine which man has the different weight by using the seesaw three times. Here is one possible method

Divide the 12 men into three groups of four: A, B, and C.

Weigh group A against group B on the seesaw. (First use)

If the seesaw balances, then the odd man is in group C. Otherwise, he is in the heavier or lighter group (A or B).

Take two men from group C and weigh them against two men from group A or B that were balanced. (Second use)

If the seesaw balances again, then the odd man is one of the remaining two men from group C. Otherwise, he is one of the two men from group C that were weighed.

Weigh one of the suspected men against any other man. (Third use)

If the seesaw balances, then the odd man is the other one. Otherwise, he is the one that was weighed.

This method works because it eliminates half of the possible candidates at each step and identifies whether the odd man is heavier or lighter by comparing him with known balanced men.

Explanation: