Reading clock faces is a tricky subject for many children, especially as they see time displayed digitally more now than ever before.
This page looks at finding lengths of time under an hour by comparing two clock faces.
There are several stages of development with this and children should be able to count up in fives confidently before starting this exercise.
At first children will probably go round the clock face counting in fives. later they might well move in 30 minute or 15 minute blocks.