When you count to 20, you start at 1 (or zero) and count on until you reach 20. Each number is one more than the last. When you count back from 20, you start at 20 and count back until you reach 1 ...

Students could take turns to pick a card with a number from 11 to 20 from a small bag. They could put their hand into another bag and pick a card saying, for example, ‘add 1 more’ and use ...