Question

There is an island with 12 islanders. All of the islanders individually weigh exactly the same amount, except for one, who either weighs more or less than the other 11.

You must use a see-saw to figure out whose weight is different, and you may only use the see-saw 3 times. There are no scales or other weighing device on the island.

Answer 1

Let's two sides of see-saw are L & R.
and assume that one person is weighing less than 11 persons of same wt
1)L(6persons) vs R(6persons)...............................................................................(1st attempt)
Either LR,so remove lowerside of see-saw's 6 persons.
2)L(3persons) vs R(3persons)..............................................................................(2nd attempt)
again,either L<R, or R<L, remove lowerside of see-saw's 3 persons.
Now remaining persons are 3.
3)Take any two persons from 3 and let them sit on see-saw..........................(3rd attempt)
a)if L=R then 3rd person is light weighted.
b)if L<R then left side person is light weighted.
c) if R<L then right side person is light weighted.