Math Background

Addition and Subtraction: When Students Ask

  • Why should I learn this?
    Show students very large numbers, like distances between planets, taken from an almanac or encyclopedia. Help them understand that addition and subtraction with large numbers is similar to addition and subtraction with 2-digit and 3-digit numbers. Encourage students to discuss the importance of knowing how to add and subtract with all kinds of numbers. What would life be like if we couldn't add or subtract?
  • Why can't I start adding or subtracting from the left?
    It is important to emphasize the impact that regrouping has on an answer. Starting from the left makes it impossible to account for those digits that have been regrouped to the next column.
  • How can I tell when I need to regroup in subtraction?
    Not all subtraction problems require regrouping. Give students practice with both types of problems. Remind them that they must subtract the bottom number from the top number. Then guide them to realize that regrouping is needed only if the top digit is smaller than the digit below it. Help students use place-value blocks to regroup 1 ten as 10 ones or 1 hundred as 10 tens.

Houghton Mifflin Math Grade 3