Lesson 8

Add and Subtract Decimals With Paper and Pencil

Est. Class Sessions: 2–3

Daily Practice and Problems

Teacher Notes
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TIMS Challenge

  1. 500 × 100 − 100 = 49,900 (largest)
    500 × (100 − 100) = 0
  2. 60 × 30 × 4 = 7200 (equal)
    6 × 3 × 400 = 7200
  3. 80 × (20 + 30) = 4000 (largest)
    80 × 20 + 30 = 1630
  4. 2/3 + 3/4 + 7/8 + 5/6 = 31/8
    4 − 2/3 = 31/3 (largest)
  5. 2 × 30 = 60 (equal)
    3 × 20 = 60
  6. 23 = 8
    32 = 9 (largest)
  7. 43 = 64 (equal)
    82 = 64
  8. 26 − 25 = 32 (equal)
    82 − (4 × 8) = 32
  9. Possible response. I changed all the fractions in the first number sentence to have the common denominator of 24.
    16/24 + 18/24 + 21/24 + 20/24 = 75/24;
    75/24 = 33/24; 33/24 = 31/8.
    Then I subtracted 4 − 2/3 = 33/3 − 2/3 = 31/3.
  10. I solve 43 = 4 × 4 × 4 = 16 × 4 = 64. Then I solve 82 = 8 × 8 = 64.

BB. Which Is Largest

Find which pairs of number sentences are equivalent. For the others, tell which is greater.

  1. 500 × 100 − 100 =

    500 × (100 − 100) =
  2. 60 × 30 × 4 =

    6 × 3 × 400 =
  3. 80 × (20 + 30) =

    80 × 20 + 30 =
  4. 2/3 + 3/4 + 7/8 + 5/6 =

    4 − 2/3 =
  5. 2 × 30 =

    20 × 3 =
  6. 23

    32
  7. 43

    82
  8. 26 − 25 =

    82 − (4 × 8) =
  1. Show or tell how you decided your answer for Question 4.
  2. Show or tell how you decided your answer for Question 7.