You have a set of 10 weights, each weight weighing 10g. There are 10 such sets. But set contains 10 weights of 9g. Determine the set of 9g weights by using a weighing scale once.

Soln: take 1 weight from set1, 2 from set2 ... 10 weights from set10. Put all of them on the weighing scale at once. 100g - weight on the scale will give the set for the 9g weights.