A class, club or party is sorted two ways, such as has a pet and has a bike, and drawn as two overlapping circles in a box. The student is given four of the numbers and finds the fifth: how many are in both, in neither, in only one, or in the whole group. They can write a number in each part of the diagram as they go.
The classic slip is adding the two circles, which counts the children in the middle twice. The working fills in the diagram one part at a time.

Numbers&Me is free. Sign up to practise more, or set it as homework in under a minute, and it marks itself.