Finding Ten More and Ten Less in Your Head for Any Number to 100

This idea walks around your house all day. Mental math keeps working when there is no paper: at a market, in a car, in the middle of a job. This is worth learning because mental math works everywhere, even with no paper in sight.

Watch for it today, and when you spot it, tell someone what you noticed and how you knew. Ask yourself where else the same idea might be hiding, then go and look.

This is a good thing for a young mathematician to know. You can do this without any tools now. Ten more than 46 is 56. Ten less than 46 is 36. Only the tens digit moves, up one or down one, and the ones digit sits perfectly still.

Practice until it comes instantly, without counting. Have someone call out numbers while you answer ten more and ten less as fast as you can say them. Speed here will help you for years.

Watch out for slowing down into counting. If you catch yourself counting one at a time, stop and look at the tens digit instead. This skill is meant to be instant, and going slow here means the idea has not quite landed yet. Do it carefully rather than quickly, and thank God for a mind that can learn.