Subtract with Mental Math

Est. Class Sessions: 2

Developing the Lesson

Part 2: Subtraction Strategies for Larger Numbers

Mental Math Strategies to Find Differences. Use a display of the Mental Math Subtraction Strategies pages in the Student Activity Book to explain the activity. After students solve each problem, have them estimate the differences to check for reasonableness. Encourage students to refer to the Subtraction Strategies Menu in the Student Activity Book Reference section and to try to use a variety of mental math strategies to find the differences. Have open number lines and base-ten pieces readily available. Students may work in pairs or work independently and then discuss strategies with partners. See Figure 4 for possible strategies for finding the exact answers and estimates.

Have students share their strategies for solving multidigit subtraction problems. If they finish early, encourage students to solve the problems in more than one way. Help students develop their own mental math strategies and understand why various subtraction strategies work before they move on to paper-and-pencil methods.

Compensation Strategies. If students use various compensation strategies, you may want to explain that when they compensate, they adjust numbers to make calculations easier. For example, in Question 5 on the Mental Math Subtraction Strategies page, Josh took away 100 instead of 99 because it was easier to subtract 100. Then he had to add one to his answer to make it the exact answer. Decide if you want to explain what compensation means or if you want to call any of the strategies compensation strategies. Students can label their strategies with their names (e.g., Josh's strategy) and refer to the Estimation Strategies for Subtraction chart to see what the strategy involves.

Adventure Book:

Student Activity Book: 744

Answers