Math Background

Lesson: Adding and Subtracting Two-Digit Numbers and Money Amounts
Developing the Concept

Begin by reviewing basic one-digit subtraction facts. When children demonstrate proficiency with these facts, explain that they will next learn how to subtract two-digit numbers.

Materials: blank transparency, overhed base-ten blocks, coins

Preparation: Draw a tens-and-ones frame on the transparency. Have children practice using a tens-and-ones frame and base-ten blocks to represent two-digit numbers.

Prerequisite Skills and Concepts: Children should have a solid grasp of subtraction of basic facts.

  • Say: You have learned to add two-digit numbers. Today you will learn how to subtract two-digit numbers.
    Using the overhead base-ten blocks, show 2 tens and 5 ones.
  • Ask: I have 2 tens and 5 ones. What number does this show? (25)
  • Ask: How can I show subtraction with base-ten blocks? (Take blocks away.) Suppose I want to take away 12. What do I need to do?
    Children may suggest that you take away 1 ten and 2 ones. Take them away and have the children count to find the difference: 1 ten and 3 ones, or 13.
  • Say: We can use a tens-and-ones frame to help us subtract 12 from 25.
    Guide children as they tell you to write a 2 in the tens column and a 5 in the ones column. Then place a 2 in the ones column and a 1 in the tens column. Explain that as in addition you begin subtracting with the ones column.
  • Ask: If I subtract 2 ones from 5 ones, how many ones are left? (3) Write 3 in the ones column.
    If I subtract 1 ten from 2 tens, how many tens are left? (1 ten) So what is 25 − 15? (13)
    Have children note that the difference is the same as the difference they got using the blocks. Replace the tens and ones with coins representing 25¢. Have children count the coins.
  • Ask: Suppose I want to subtract 12¢ from 25¢. How can I show the subtraction with coins?
    Children may suggest taking 1 dime and 2 pennies away. Then count to see how many dimes and pennies are left. Have children determine that there are 1 dime and 3 pennies left, or 13¢.
  • Say: You can subtract money amounts just as you subtract counting numbers. Just remember to place the cents symbol after each money amount.
    Write 25¢ and 12¢ in the tens-and-ones frame. Relate the dimes to the "tens" and the pennies to the “ones.” Beginning with pennies, work through the subtraction. Explain that when subtracting money amounts that are less than a dollar, children need to write the cents symbol after each amount. Conclude by referring the children to the original example and noting that the only difference between subtracting money amounts and subtracting counting numbers is the cents symbol. Repeat the activity using examples that do not require regrouping.

Wrap-Up and Assessment Hints
Give children addition and subtraction exercises that include adding or subtracting multiples of ten and adding or subtracting one-digit numbers. Suggest that they choose any method they wish to solve them—paper and pencil, base-ten blocks, or basic facts. You can assess children's progress informally by asking them to explain why they chose their method.

Houghton Mifflin Math Grade 1