Ten more and ten less

When you add 10 to a number, only the tens digit changes. It goes up by one. 10 more than 37 is 47. The ones stay the same.

When you take away 10, the tens digit goes down by one. 10 less than 37 is 27. This is a quick trick you can do without counting every number.

Try it with bigger numbers too. 10 more than 95 is 105, because 9 tens and one more ten make 10 tens, which is 100.