Washbasin puzzle

 You have a jug which will carry 7 litres of water, and a washbasin which holds exactly 20 litres. By pouring the contents of the jug into the basin, or by emptying the basin when it is full.

 How can we end up with exactly 4 litres of water in the washbasin?

Solution:

To end up with 4 litres in the washbasin you need to pour in 12 full jugs of water (84 litres in total) emptying the washbasin each time it gets full to capacity.

This is an example of a modular arithmetic problem and it is not a unique solution. Any multiple of 7 which has a remainder 4 when divided by 20 will also be a solution of the problem.