Lesson Plan - Get It!
If your family was baking cookies, and you didn't have enough of an ingredient, what would you do? You might drive to a store or you might borrow from your neighbor! How is borrowing used in math?
Subtraction is used to find the difference between two numbers.
You have probably learned that the larger number is usually the starting number in subtraction. Sometimes, you will need to regroup, or borrow, from another number before you can subtract.
- When is regrouping needed in a subtraction problem?
Take a look at two phrases that will help you remember whether you need to regroup or not.
Just like with addition and subtraction of any number, start in the ones place.
"More on the top? No need to stop!"
Subtract 44 from 85:
Look at the ones place. 5 is on the top and 4 is below the 5. Since 5 is greater than 4, subtract and move to the tens place. 8 is on top and 4 is below the 8. Since 8 is greater than 4, subtract straight down. This problem did not need regrouping.
"More on the floor? Go to the neighbor and borrow 10 more!"
Subtract 58 from 73:
Look at the ones place. 3 is on the top. 3 is smaller than 8, which is the number below it. You need to borrow from the tens place. Borrowing from the tens place will add value to the ones place, without changing the overall value of the number.
Since you cannot subtract 8 from 3, you will need to borrow a 10 and add it to the ones place. The 7 tens become 6 tens, and the 3 ones become 13 ones. We show this by adding 3 ones and 10 ones, which is 13 ones. You can see in the example below this is shown by placing a "1" in front of the value in the ones place. Now it is 13.
Now, you can subtract both place-value spots, starting with the ones:
It is helpful if you can look at a subtraction problem and decide whether or not regrouping is needed. If the value on the top is less than the value below it, you will need to regroup. You cannot take away more than what you start with. If you had $7, could you give someone $9? No, because you only have $7!
Decide whether or not these problems would need regrouping, but do not solve them! Discuss with your parent or teacher:
- 65 - 24 =
- 99 - 19 =
- 71 - 17 =
- 43 - 18 =
Did you choose Problems C and D as the ones that need regrouping? Great! If not, go back and review them with your parent or teacher.
Continue on to the Got It? section to practice two-digit subtraction with regrouping by playing interactive games.