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.