Math Background

Three-Digit Addition and Subtraction with Regrouping: When Students Ask

  • How do I know when I need to regroup to solve an addition or subtraction problem?
    In an addition problem, you need to regroup if there are more than 10 ones or 10 tens. In a subtraction problem, you need to regroup if there are not enough ones or tens to subtract.
  • How can I keep track of which numbers I have regrouped?
    In addition, lightly write the ten you regrouped from the ones, or the hundred you regrouped from the tens above the appropriate digit. Then remember to add those lightly-written numbers when you add the tens and the hundreds. In subtraction, cross out the digits of the original number as you regroup tens or hundreds in the top number. Add the 10 ones you regrouped to the digit in the ones place and the 10 tens you regrouped to the digit in the tens place. Write the new numbers above the original digits in the appropriate columns.
  • What should I remember about adding or subtracting money?
  • Why is estimating important?
    Estimating helps you get a close idea of the answer to a problem without solving the problem. Sometimes it is not important to have an exact answer. An estimate will do. Sometimes you can check to make sure that your answer is reasonable by doing a quick estimate of the answer.
  • What do I need to remember when I estimate?
    When estimating the sum or difference of two 3-digit numbers, remember to figure out which hundreds the numbers in the exercise are nearest to. This is called “rounding” to the nearest hundred. Then you can add or subtract to find the estimate.

Houghton Mifflin Math Grade 2