Estimating Two-Digit Sums and Differences

Purpose

To estimate two-digit sums and differences

Materials

None

Overview

Say 2 two-digit numbers aloud that do not involve making a new group of ten when added together, such as 23 and 31. Ask children for an estimate of the sum. Repeat with a few pairs of numbers that do not involve regrouping. You may suggest rounding to the nearest ten as a strategy for children to use.

Next, say 2 two-digit numbers aloud that do involve regrouping. If a student responds by just adding the tens digits, accept the answer, but ask if they can refine the estimate. Help them recognize that if the ones digits make a sum of 10 or more, they can get a better estimate by adding 10 to the previous estimate. Remind children that using a compatible numbers strategy can sometimes be helpful. Ask them to try this strategy for estimating sums with problems such as 74 + 23, 52 + 23, and 75 + 15.

About the sequence

Part 1 begins with estimating the sum of 2 two-digit numbers that do not involving making a new group of ten (regrouping). Part 2 evolves to using 2 two-digit numbers that do involve regrouping. The extension involves a mix of regrouping and non-regrouping pairs of two-digit numbers with sums that may be greater than 100.

Part 1

I’m going to say 2 two-digit numbers. I’d like you to estimate their sum.

Examples:

11 + 18 (10 + 20 = 30)
22 + 46 (22 + 50 = 70)
38 + 31 (40 + 30 = 70)
43 + 52 (40 + 50 = 90)
72 + 17 (70 + 20 = 90)
27 + 42 (30 + 40 = 70)
21 + 68 (20 +70 = 90)

If children need more practice or are enjoying their mastery, repeat. If children are ready to move on, go to Part 2.

Part 2

I’m going to say 2 two-digit numbers. I’d like you to estimate their sum.

Examples:

19 + 17 (20 + 20 = 40)
26 + 18 (30 + 20 = 50)
34 + 48 (30 + 50 = 80)
19 + 38 (20 + 40 = 60)
56 + 22 (60 + 20 = 80)
33 + 49 (30 + 40 = 70)
21 + 19 (20 + 20 = 40)

If children need more practice, repeat. Or, when children seem excited for a new challenge, move to the extension.

Extension

I’m going to say 2 two-digit numbers. I’d like you to estimate their sum.

Examples:

67 + 42 (70 + 40 = 110)
87 + 26 (90 + 30 = 120)
77 + 89 (80 + 90 = 170)
51 + 69 (50 + 70 = 120)
86 + 97 (90 + 100 = 190)
62 + 59 (60 + 60 = 120)