When subtracting 11 from a number often the easiest way is to subtract 10 and then count back one. Some people prefer to subtract the unit first and then the ten. The same can be said for subtracting 21: subtract 20 and count back one. These pages are good practice for children just getting to grips with tens and units and as they are comparatively easy are excelent confidence boosters. Subtract 11 or 21 from 2-digit numbers