Math Background

Two-Digit Subtraction With Regrouping: When Students Ask

  • Will I always regroup when I subtract?
    Explain to children that they will not always regroup. To decide whether or not they should regroup, they should look at the two numbers in the ones column. If the number on top in the ones column is less than the number they are subtracting from it, they should regroup. Remind children that if they have to regroup, they must regroup 1 ten as 10 ones. Explain that if the number on top in the ones column is larger than the number they are subtracting from it, they can subtract without regrouping. Also remind them that they will not take 1 ten from the tens column if they do not need to regroup.
  • How can I subtract two-digit numbers correctly?
    Explain to children that they first need to decide whether or not they need regroup so they should check the number of ones in the ones column carefully. Remind children that it is necessary to record each step as they regroup. First, they cross out the number of tens and write the new number of tens above the tens column. Then they cross out the number of ones at the top of the ones column and write the new number of ones. Finally, they subtract the ones column, and then they subtract the tens column.
  • Is regrouping for subtraction different from regrouping for addition?
    Yes. In addition, children add the ones and regroup the ones to make a ten if there are more than 10 ones. In regrouping for subtraction, they take away the ones. If there are not enough ones to subtract, they have to regroup a ten to make 10 ones and add those ones to the number of ones at the top in the ones column. Further explain that in regrouping for addition, they regrouped ones to make a ten; for subtraction, they take away a ten and regroup it to make 10 ones.
  • Why do I need to know this?
    Explain to children that they will be using subtraction all through their lives. Tell children that they will need to know how to regroup in subtraction when they learn division. Point out that one very important use for subtraction will be keeping track of their money in a checkbook.
  • Why should I know how to estimate a difference?
    Tell children that it is not always necessary to have an exact answer. When they go to the store, they will want to know approximately how much money they should get in change from their purchase. If they can estimate quickly, they can tell if their change is close to the amount they should have received, or whether they should check the amount more carefully.

Houghton Mifflin Math Grade 2