44 Speed Math for Kids
then multiply the answer by 10, or, you could say, you double the
number and add a 0.
For instance, to multiply 7 by 20, you would double it (2 × 7 = 14)
and then multiply your answer by 10 (14 × 10 = 140). To multiply 32
by 20, you would double 32 (64) and then multiply by 10 (640).
Multiplying by 20 is easy because it is easy to multiply by 2 and it is
easy to multiply by 10.
So, it is easy to use 20 as a reference number.
Let us try an example:
23 × 21 =
Twenty-three and 21 are just above 20, so we use 20 as our reference
number. Both numbers are above 20, so we put the circles above.
How much above are they? Th ree and 1. We write those numbers
above in the circles. Because the circles are above, they are plus
numbers.
+3 +1
20 23 × 21 =
We add diagonally:
23 + 1 = 24
We multiply the answer, 24, by the reference number, 20. To do
this, we multiply by 2, then by 10:
24 × 2 = 48
48 × 10 = 480
We could now draw a line through the 24 to show we have fi nished
using it.
Th e rest is the same as before. We multiply the numbers in the
circles:
brent
(Brent)
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