Subtraction Strategies

Est. Class Sessions: 1

Summarizing the Lesson

Continue practicing using strategies to solve comparison problems by playing Guess My Strategy. Display the Invented Strategies chart and the following problem you prepared prior to the lesson. See Materials Preparation.

There are 17 pencils and 9 crayons in a box. How many more pencils than crayons are in the box?

Allow sufficient time for student pairs to solve the problem using one of the strategies on the Invented Strategies chart. Have connecting cubes, 100 Charts, number lines, and ten frames readily available. Encourage students to use more than one strategy to solve the problem. Explain that this helps them check the reasonableness of their answers.

When students are finished, select one student pair to present their strategy to the class by showing or telling how they solved the problem without naming the strategy. Other class members try to guess which strategy they used (counting-on, counting-back, using doubles, using 10).

The following is a list of some strategies students may use:

I know that 9 + 9 is 18, so 9 + 8 = 17. Guess my strategy. (using doubles)
I know that 10 + 8 = 18, so 9 + 8 = 17. (using ten)
I started at 9 and I said: 10, 11, 12, 13, 14, 15, 16, 17. [holding up 8 fingers] (counting up)
I started at 17 and counted: 16, 15, 14, 13, 12, 11, 10, 9. [holding up 8 fingers] (counting-back)
I know that 9 + 8 = 17, so 17 − 9 = 8. (related addition facts or fact families)
I drew a number line, started at 17 and moved back 9 jumps. (counting back on the number line)

Assign the Use Strategies pages in the Student Activity Book to assess students' abilities to use various strategies to solve comparison problems. Students have to explain the strategy they used to solve each problem. At this point, it is acceptable for students to explain how they solved the problem by using diagrams, pictures, or words without naming the strategy (counting on, making 10, using doubles). Have connecting cubes, 100 Charts, number lines, and ten frames readily available.

Assign the Use Strategies pages with the Feedback Box in the Student Activity Book to assess students' abilities to represent subtraction using stories, drawings, diagrams, counters, number sentences, number lines, or ten frames [E2]; solve word problems (e.g., compare) involving two whole numbers whose sum is between 10 and 20 [E3]; use mental math strategies and reasoning strategies (e.g., using doubles, using ten, making ten) to solve addition problems with sums between 10 and 20 and the related subtraction problems [E5]; use strategies that apply the properties of addition (e.g., turn-around, compose and decompose numbers) to solve addition and subtraction problems [E6]; find the unknown whole number in an addition or subtraction equation relating three whole numbers [E7]; know the problem [MPE1]; find a strategy [MPE2]; and show my work [MPE5].