Repeated Subtraction Background Information for Teachers, Parents and Caregivers
This page provides information to support educators and families in teaching K-3 students about repeated subtraction. It is designed to complement the Repeated Subtraction topic page on BrainPOP Jr.
This movie will introduce repeated subtraction as a way to explore division. You may want to explore the Making Equal Groups movie in conjunction with this one to reinforce or extend the topic. We recommend using number lines or hundred charts and employing different subtraction strategies, such as counting back or skip-counting back, to offer children multiple ways to solve number sentences. You may wish to pause the movie as you watch together and have children solve the examples in the movie before Annie and Moby do.
Review with children that when they divide, they take a larger group and split it into smaller groups. We can solve division number sentences by subtracting the same number over and over again. Pose a story problem such as, “A pizza has 9 slices and 3 friends share it. How many slices does each friend get?” You may want to draw pictures or use props to help children visualize the problem. Guide children to write the division sentence that solves the problem 9 ÷ 3. Explain that we can solve the sentence by starting at 9 and subtracting 3 over and over again until we get to 0. Be clear that the number of times we subtract 3 is the answer. Then subtract together, using tally marks to keep track of the number of times you subtract: 9 – 3 = 6, 6 – 3 = 3, 3 – 3 = 0. We subtracted 3 times, so 9 ÷ 3 = 3. Repeat the activity with other number sentences or story problems.
Introduce children to other strategies they can use in order to solve division sentences. Pose another story problem such as, “Annie needs to sell $30 worth of chocolate bars for a fundraiser. Each bar costs $5. How many bars will she have to sell?” Explain that we can use a hundred chart or a number line to solve. We start at 30 and go back by 5s until we get to 0 and then count the number of times we counted back. Model how to solve the problem on a hundred chart or number line. Show how we go back by 5s for a total of 6 times. So, the answer is 30 ÷ 5 = 6. Annie needs to sell 6 chocolate bars to make her goal.
We can also use what we know about multiplication facts to solve division problems. Review related facts and fact families with children, such as 2 x 7 = 14, 7 x 2 = 14, 14 ÷ 2 = 7, 14 ÷ 7 = 2. Point out how the number sentences use the same numbers and inverse, or opposite, operations. You may want to use a fact triangle to help children visualize. Encourage them to use related multiplication facts to solve division facts.
Practice solving different number sentences and invite children to come up with their own story problems. Use models, counters, pictures, and other strategies to help children visualize and check their work.
