This puzzle is asked in most of the interview questions and sounds tough whenever you hear it first time.
There are 10 buckets each containing 100 small balls weighing 1 gram except the one bucket which contains balls weighing 0.9 gram. All the balls are of same colour and looks same. You are given a weighing machine which displays the weight in grams. You have to use it only once and you have to find out which bucket contains 0.9 gram balls.

The solution is easy. Mark the buckets from 1 to 10 and pick one ball from 1 bucket.
2 balls from 2nd bucket.
3 balls from 3rd bucket and so on.
till
10 balls from 10th bucket.

Now weigh them on the weighing machine and check your reading.
let’s say it displays 54.9 grams.
Ideally it should have been 55 grams if all the balls were of 1 gram so since there is a difference of 0.1 gram in the solution therefore

bucket no 1 contains the balls weighing 0.9 gram.
Similarly it can be predicted depending upon the weight displayed by the weighing balance.