Chapter 4.5 (Mathematics 5)

Estimating products using friendly factors

Estimating products using friendly factors

Group Activity 4

Estimate the products of 185 and 58 using friendly factors.

In pairs:

  1. Estimate 185 to the nearest hundred.
  2. Estimate 58 to the nearest ten.
  3. Multiply the two estimate factors.
  4. Discuss the results and share with other groups.

Note

  1. Friendly factors are also called compatible numbers.
  2. Friendly factors make multiplication mental and easy.

Example 5

Estimate the products of 357 and 53.

  • 357 is estimated to 400 to the nearest hundred.
  • 53 is estimated to 50 to the nearest ten.
  • 400 × 50 = 20 000.

Note

It is easier to multiply 400 by 50 than 357 by 43.

Exercise E

      • 273 × 81
      • 522 × 47
      • 827 × 33

      E2

      Estimate the products by using friendly factors.

      1. 108 × 86
      2. 397 × 39
      3. 654 × 74
      4. 949 × 86
      5. 199 × 81
      6. 766 × 34
      • 21 000
      • 10 000
      • 30 000
      • 719 × 32
      • 487 × 19
      • 563 × 44