# Lesson Plan

# Adding and Subtracting Tens Background Information for Teachers and Parents

This page contains information to support educators and families in teaching K-3 students about adding and subtracting tens. The information is designed to complement the BrainPOP Jr. movie Adding and Subtracting Tens. It explains the type of content covered in the movie, provides ideas for how teachers and parents can develop related understandings, and suggests how other BrainPOP Jr. resources can be used to scaffold and extend student learning.

As children begin working with larger numbers, teach them strategies and shortcuts that help them manipulate numbers more easily. Help them understand that they can use basic facts they already know to solve more complicated number sentences and problems. We suggest reviewing the Making Ten and Doubles movies before exploring this topic. We also recommend that children use counters, base-ten blocks, number lines, and hundred charts to practice adding and subtracting tens.

Use base-ten blocks or counters to show 40 and 20. Then have children add the two numbers together. Try to add them in different ways. For example, you can skip-count each tens rod or you can start with 40 and add on two tens: 50, 60. You can also use a number line or a hundred chart and add on two tens. If you are using a hundred chart, be sure children notice that they jump down two rows for two tens. Remind them that each row on a hundred chart has ten numbers. Explore a variety of strategies, and write the number sentence to show how you added: 40 + 20 = 60. You may wish to write the sentence horizontally and vertically.

Explain to children that when they are working with tens, they can hide the zeroes to make it easier to add, and then bring back the zeroes at the end. So 50 + 30 can become 5 + 3, which is a basic number fact. Since 5 + 3 = 8, then 50 + 30 = 80. Try solving other number sentences together where children add tens by hiding the zeroes.

Encourage children to use doubles facts to help them solve number sentences. For example, many children know 3 + 3 because it is a basic doubles fact. They can use this fact to solve 30 + 30. Just remind them to bring back the zeroes after they solve.

Remind children that when they subtract using manipulatives, they take away pieces. Use base-ten blocks to show 90. Then, take away 20. How many are left? Write the number sentence 90 – 20 to show what happened. Then use a number line, a hundred chart, or the base-ten blocks to solve the problem. Remind children that they can hide the zeroes to help solve the problem. Since 9 – 2 = 7, they know that 90 – 20 = 70.

Once children are familiar with adding and subtracting tens, they can use those strategies to help them solve other number sentences. Show the number sentence 20 + 11. What is the sum? Write the number sentence 20 + 10 and have children solve. Explain that in 20 + 11, they are adding 11 and not 10. Since 11 is one more than 10, the sum of 20 + 11 should be one more than 20 + 10. Thus, 20 + 10 + 1 = 31. Practice this strategy with other number sentences.

Show the number sentence 50 + 9. What is the sum? Some children may remember that 50 + 10 = 60 (or remember that 5+ 1 = 6, therefore 50 + 10 = 60). How can they use this fact to solve 50 + 9? Remind children that in 50 + 9, they are adding 9 and not 10. Since 9 is one less than 10, the sum of 50 + 9 is one less than 50 + 10. Thus, 50 + 10 – 1 = 59. Practice this strategy with other number sentences.

Now show the number sentence 60 – 11. What is the difference? Write down the number sentence 60 – 10 and have children solve. Some children may be able to quickly solve 60 – 10 = 50 (or recall that 6 – 1 = 5, thus 60 – 10 = 50). How can they use this number sentence to solve 60 – 11? Remind children that in 60 – 11, they are subtracting 11 and not 10. Since 11 is one more than 10, the difference of 60 – 11 is one less. Thus, 60 – 10 – 1 = 49. Practice this strategy with different number sentences. We recommend using hundred charts and base-ten blocks to help children. This strategy will be useful later when they start regrouping while subtracting.

Show the number sentence 70 – 9. What is the difference? Write down the number sentence 70 – 10. Children may recall that 70 – 10 = 60. Have them use this number sentence to solve 70 – 9. Since 9 is one less than 10, the difference of 70 – 9 should be one more than 70 – 10. Thus, 70 -9 = 61. Practice solving other number sentence without regrouping.

Help children understand that adding and subtracting tens is a useful strategy that can be used to solve other number sentences. With plenty of practice, children can become comfortable solving two-digit addition and subtraction sentences without feeling overwhelmed or daunted. They can use their knowledge to apply to more complex number sentences, and are well on their way to becoming math pros!