Useful Mental Subtraction Strategies:

Model Subtraction: use physical objects to show "take away". Students can then count the objects.

Counting Back: count back from the larger number, i.e. 18-3 is "18... 17, 16, 15".

Link with Addition: to solve 6 - 3, think 3 and 3 is 6.

Go down to ten: for 15 - 8 think 15 take 5 is 10, take 3 more is 7.

Do not introduce the vertical subtraction algorithm until students have developed strong mental strategies!