1.NBT.C.5

Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.

Find 1 more, 1 less, 10 more, or 10 less. To find 10 more or 10 less, the units stay the same, only the tens place changes. Do not count. For example, when finding 10 less than 46, do not count 46, 45, ..., 36.