Examples, solutions, videos, worksheets, games and activities to help Grade 3 students learn estimation strategies for addition and subtraction.

In this lesson, we will learn how to apply estimation strategies to predict sums and differences of two 2-digit numerals in a problem solving context. Some of the estimation strategies are: Front-End Strategy and Closest-Ten Strategy (or rounding).

Estimating sums and differences is valuable because it helps predict an answer and check a calculation. When using estimation in a problem solving context, there are important things to keep in mind. What is best, an exact answer or an estimate? How important is it for the estimate to be close to the exact value? Is it better to have a low or high estimate?

The front-end strategy is a method of estimating computations by keeping the first digit in each of the numbers and changing all the other digits to zeros. This strategy can be used to estimate sums and differences. Note that the front-end strategy always gives an underestimate for sums.

*Example:*

When estimating 77 - 24

Think: 77 -> 70 and 24 -> 20.

70 − 20 = 50. The estimate is about 50.

*Example:*

Karen is taking piano lessons and her piano teacher asked her approximately how much time she practiced on Saturday and Sunday. Karen knew she practiced 43 minutes on Saturday and 56 minutes on Sunday.

To find an estimate for 43 + 56, Think: 43 -> 40 and 56 -> 50. 40 + 50 = 90.

Karen could say she practiced about 90 minutes.

This video shows how to estimate addition using front-end estimation.

Front End Estimation with Adjustment

Strategy for solving addition word problems.

Write each number as an approximation by rounding the number to the closest ten.

*Example:*

When estimating 77 − 24

77 is 3 away from 80, so we round to 80.

24 is 4 away from 20, so we round to 20.

80 − 20 = 60. The estimate is about 60.

*Example:*

Erin has 83 colored beads to make necklaces for her friends. She uses 37 beads to make a necklace for Julia. About how many beads does Erin have left?

83 round to 80. 37 round to 40

The estimate is 80 − 40 = 40 beads

How to estimate sums by rounding and by compatible numbers

Estimation strategies require the use of rounding techniques. If you need help to review rounding techniques then see the following videos.

Description of Rounding and Estimation.

Use estimation and rounding to deal with problems.

Front-End Estimation

3,025 + 498 + 32

613 + 91 + 1,231

Clustering

51, 48, 53, 50, 47

93, 103, 99, 97, 101, 104

Compatible Numbers

73.8 + 7.9

22.6 + 3.04

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