ffirs.indd

40 Speed Math for Kids
I write the substitute numbers in pencil above or below the actual
numbers in the problem. It might look like this:
137 × 456 = 62,472
2 6 3
Is 62,472 the right answer?
We multiply the substitute numbers: 2 times 6 equals 12. Th e digits
in 12 add up to 3 (1 + 2 = 3). Th is is the same as our substitute
answer, so we were right again.
Let’s try one more example. Let’s check if this answer is correct:
456 × 831 = 368,936
We write in our substitute numbers:
45 6 × 831 = 368,936
6 3 8
Th at was easy because we cast out (or crossed out) 4 and 5 from
the fi rst number, leaving 6. We cast out 8 and 1 from the second
number, leaving 3. And almost every digit was cast out of the answer,
3 plus 6 twice, and a 9, leaving a substitute answer of 8.
We now see if the substitutes work out correctly: 6 times 3 is 18,
which adds up to 9, which also gets cast out, leaving 0. But our
substitute answer is 8, so we have made a mistake somewhere.
When we calculate it again, we get 378,936.
Did we get it right this time? Th e 936 cancels out, so we add
3 + 7 + 8, which equals 18, and 1 + 8 adds up to 9, which cancels,
leaving 0.
Th is is the same as our check answer, so this time we have it right.
Does this method prove we have the right answer? No, but we can
be almost certain.
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