Identifying these “numbers before” is less straightforward than it may seem. Many children find it hard to identify the number one less than, for example, 300 or 2,000. For children learning to count, going forwards is not easy either. Many initially get stuck on the numbers ending in nine and young children have been known to say that 19 is followed by “tenteen” or 29 by “twenty-ten”.

Numbers ending in nine have various associations, from 19 nervous breakdowns to 99, an ice cream with a Flake. But for a collective word I am indebted to a former pupil who called them “nearly numbers”. I suspect this stemmed from his preoccupation with being nearly 10.

Many children are aware that you can add nine to a number by adding 10 then subtracting one. The national framework for teaching mathematics refers to this strategy as “adjusting”. Year 1 pupils should be able to add nine to a single digit number by adding 10 and subtracting one. Older children should extend the strategy to add on to larger numbers and should adapt it to add, for example, 19 or 29.

But the method really comes into its own with subtraction. Anyone who has seen a child attempt to do 200-99 by writing the numbers underneath each other and using decomposition will agree that there must be an easier way.

Calculating with the nearly numbers * Discuss methods for adding nine to numbers.

* Extend this to find methods for adding 19, 29...99...199...

* Look at subtraction of numbers ending in 9 or 99.

Nearly numbers in context * Search for nearly numbers. They are very common, especially in the context of money.

* Video or book catalogues make a good starting point for money calculations - many prices end in 9 or 99 .

* Use adverts for second-hand cars as a source of bigger nearly numbers.

* House prices can be used for rounding larger numbers.

Jenny Houssart is a research fellow at the Open University Centre for Maths.