"to replace a number with another number that tells about how many or how much". what is ths operation called?

This operation is called rounding. 

For example, if I scored 97.9 points on a math test, I could round it up to 98. Or, if I scored 92.1 points, then I could round it down to 92. In both cases, the real math scores (both of which are decimals) are replaced by different, whole numbers.