To estimate a number in math, round the last digit of a number up or down to the nearest zero number. This is sometimes referred to as the nearest 10.
An example of estimating would be 123 + 16. If the last number is less than five, then round down to a zero number, in this case 120. If the last number is five or greater than five, then round to the higher zero, in this case 20. Therefore, the problem would become 120 + 20 = 140. Estimation is commonly used in multiplication, division, algebra and other mathematical disciplines.