Concepts

Estimating a sum or difference means finding a solution that is close to the exact answer. The goal is to round the addends, or minuend and subtrahend, to numbers that can easily be added or subtracted using mental math. Estimation can be used to check for the reasonableness of an answer. 

Rules

Step 1: To round the numbers to the nearest whole number, consider the place value you are rounding the number to.
If the digit in that place value is followed by 5,6,7,8, or 9, round the number up.
If the digit in that place value is followed by 0,1,2,3, or 4, round the number down.
 
Step 2: Add or subtract accordingly.

Example

The height of the first floor of a house is 15.50 meters and the height of second floor is 13.75 meters. Approximately what is the difference of heights between first and second floor? 

Solution

Difference means subtraction so estimate 15.50 - 13.75
Round the quantity of both measurements to the nearest whole number and then subtract.
15.50 (Round to nearest whole number) = 16 
(15.50 rounds up to 16 since the number in the tenth’s place is equal to 5.)

13.75 (Round to nearest whole number) = 14
(13.75 rounds up to 14 since the number in the tenth’s place is greater than 5.)
       
 Difference = 16 - 14 = 2 

Prerequisite Skills

Estimate sum by rounding (3.2.4)

Estimate Differences (3.3.3)