1.NBT.5 - Use place value understanding and properties of operations to add and subtract

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

CC.1.NBT.5 Use place value understanding and properties of operations to add and subtract. Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.