Subtracting with Regrouping Background Information for Teachers, Parents and Caregivers
This page provides information to support educators and families in teaching K-3 students about subtracting with regrouping. It is designed to complement the Subtracting with Regrouping topic page on BrainPOP Jr.
Before exploring this topic, we highly recommend reviewing movies in the Addition and Subtraction unit. Children should be comfortable employing different strategies to solve number sentences. This movie will explore subtracting with regrouping, specifically with equations in which a one-digit number is subtracted from a two-digit number. This can be a tricky topic for some children, so we recommend using base-ten blocks or counters to model number sentences. We encourage you to pause the movie and have children solve the problems posed in the movie.
Pose a number story or a number sentence that requires regrouping to solve, such as 16 – 9. Review with children that when they subtract using two-digit numbers, they must subtract the ones place first. Some children may remember that they should subtract a smaller number from a larger number, so how can they subtract 6 – 9? Remind children that when they regroup, they gather ten ones and trade it in for a ten. They can also take a ten and trade it in for ten ones. You may want to use base-ten blocks to demonstrate this concept. Explain that we can borrow a ten and turn it into ten ones in order to solve the number sentence. We can show the number 16 with a tens rod and six ones cubes, and we can also turn the tens rod into ten ones cubes and show the number using sixteen ones cubes. Now we can easily subtract 9 ones from 16 ones. So 16 – 9 = 7. Repeat this activity again to show 12 – 5 or 17 – 8.
Challenge children to solve the number sentence 35 – 8. Guide them to look at the numbers in the ones place first. They must regroup in order to solve. Use base-ten blocks to model 35. They can borrow a ten and turn it into ten ones so now there are two tens rods and fifteen ones cubes. Then they can easily subtract 8 ones cubes. Guide them to write 7 in the ones place under the answer bar. Then have them subtract the tens place. Since we regrouped a ten, there are two tens left. Guide them to write 2 in the tens place under the answer bar. Therefore, 35 – 8 = 27. Repeat the activity again with other number sentences.
Encourage children to check their work after they subtract. They can use addition to check their subtraction. So, for example, to check 35 – 8 = 27, they can use the addition sentence 27 + 8 = 35.
Walk children through solving a number sentence without using counters. Pose the number sentence 93 – 6. You may want to write the sentence horizontally and have children rewrite the sentence vertically. Make sure they line up the place value columns correctly. To solve this equation, they must regroup a ten. Show them how to cross out the 9 and write 8 next to it. Then they can add 10 to the 3 in the ones place. They can cross out the 3 and write 13 next to it. What is 13 – 6? Children can employ different strategies (such as making a ten, or using what they know about doubles) to solve 13 – 6 = 7. Have them write 7 in the ones place under the answer bar. Now they can subtract the tens place. They have 8 tens, so guide them to write 8 in the tens place under the answer bar. Therefore, 93 – 6 = 87. They can use 87 + 6 = 93 to check their work.
Give children plenty of practice to solve number sentences with regrouping. Encourage them to use different strategies to solve problems and use base-ten blocks or counters. Then transition them out of using counters. Be sure to guide them through regrouping and encourage them to check their work by using the inverse operation.
