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104 Speed Math for Kids
multiply our answer of 64.2 by 100 to get 6,420. Multiplying the
circled numbers, 4 times 3, gives us 12.
6,420 + 12 = 6,432 Answer
Th at was much easier than using 100 as the reference number.
Here is another problem using 200 as our base reference number.
189 × 77 =
We use 200 and 80 as our reference numbers. Eighty is 0.4 of 200.
(200 × 0.4) 189 × 77 =
–11
–3
–4.4
We multiply 11 by 0.4 to get 4.4. (We simply multiply 11 times
4 and divide the answer by 10.) We write 4.4 below the 11. We
now subtract 4.4 from 77. To do this we subtract 5 and add 0.6.
Seventy-seven minus 5 is 72, plus 0.6 is 72.6.
72.6 × 100 = 7,260
We multiply 7,260 by 2 to get 14,520. (We could double 7 to get
14 and 2 times 26 is 52, making a mental calculation easy.) Our
work so far looks like this:
(200 × 0.4) 189 × 77 = 7,260
–11
–3
14,520
–4.4
Now we multiply the numbers in the circles.
11 × 3 = 33
Add 33 to 14,520 to get 14,553. Here is the calculation worked out
in full.