Display the following problems and read them aloud:
Guide students to the notion that strategies used for the subtraction math facts can be generalized to two-digit numbers. Then discuss possible strategies, or combined strategies, for solving these problems.
- Choose a problem to solve in your head and explain the strategy you used. (Students may use thinking addition, using tens, or their own strategy to solve the problems. Discuss a variety of strategy solutions for each problem.)
- Was anyone able to use ten to solve a problem? Which one or ones? (To solve 25 − 9, students might count up 1 to make 10, and then find it easier to add 15 more to make 25. The difference, when counting up 1 + 15 more, is 16.)
- Did thinking addition help you to solve one of the problems? Which one or ones? (To solve 33 − 20, students might think 20 + what number equals 33? 20 + 10 more is 30, and then adding 3 more is 33, and 10 + 3 = 13.)
- How is solving 33 − 20 like taking two tens from 33? (20 is two tens. You can take one ten from 33 and get 23. Then you take another ten from 23 and get 13.)
