ffirs.indd

(Brent) #1

188 Speed Math for Kids
By direct multiplication, we have:
8 × 20 = 160
8 × 7 = 56
160 + 56 = 216
Now using two reference numbers, we have:
(10 × 3) 8 × 27 = 210
–2
–3



  • 6
    –6
    216 Answer
    Th is time the calculation was easier using two reference numbers.
    Is it diffi cult to use direct multiplication when we have a reference
    number of 60, 70 or 80? Let’s try it for 67 times 67. We will use 70
    as our reference number.
    70 67 × 67 =
    –3
    –3
    We subtract 3 from 67 to get 64. Th en we multiply 64 by our
    reference number, 70. Seventy is 7 times 10, so we multiply by 7
    and then by 10.
    Seven times 60 is 420 (6 × 7 = 42, then by 10 is 420).
    Seven times 4 is 28. Th en, 420 + 28 = 448, so 7 times 64 is 448, and
    70 times 64 is 4,480.
    Multiply the numbers in the circles: 3 × 3 = 9. Th en:
    4,480 + 9 = 4,489 Answer
    Even if you go back to using your calculator for such problems, you
    have at least proven to yourself that you can do them yourself. You
    have acquired a new mathematical skill.

Free download pdf