Adding and Subtracting Multiples of Ten

In the first part of the lesson, students filled orders by combining bags of 10 and 20. Tell students that now Maria gets marbles only in bags of ten.

Display 6 trains of ten connecting cubes and ask students to count them by tens with you. Tell students that you are taking away 4 trains of ten connecting cubes. Count the number of cubes you are taking away and the number remaining by tens.

Have desk number lines, connecting cubes, and 100 Charts readily available. Read the following problem and have students work in pairs to solve the problem and write a number sentence:

Maria had 50 red marbles. She sold 40 of them. How many red marbles did Maria have left?

After they solve the problem, have students share their strategies. See Figure 3 for example representations. Make sure your discussion includes the following possibilities:

• Skip count from 40 to 50 by tens: 40, 50.
• Show five stacks of ten connecting cubes and take away four stacks.
• Write five tens and cross out four tens.
• Show five full ten frames and cross out four.
• Start at 50 on the 100 Chart and count back four rows: 40, 30, 20, 10.
• Start at 50 on the class numberline and hop back 40.

Direct students' attention to the Selling Bags of Marbles pages in the Student Activity Book. Explain that student pairs are going to solve problems about selling bags of marbles packaged in tens. For Question 1, students finish Linda's and Joey's strategies for finding the answer and write a number sentence that describes the problem. For Questions 2–3, students use either Linda's or Joey's strategy and write a number sentence for each problem.

Upon completion, have students explain their strategies and number sentences. Ask them to explain any tools they used.