ffirs.indd

36 Speed Math for Kids
Five is our substitute for 14.
We now do the same calculation (multiplication) using the substitute
numbers instead of the original numbers:
3 × 5 = 15
Fifteen is a two-digit number, so we add its digits together to get our
check answer:
1 + 5 = 6
Six is our check answer.
We add the digits of the original answer, 168:
1 + 6 + 8 = 15
Fifteen is a two-digit number, so we add its digits together to get a
one-digit answer:
1 + 5 = 6
Six is our substitute answer. Th is is the same as our check answer, so
our original answer is correct.
Had we gotten an answer that added to, say, 2 or 5, we would know
we had made a mistake. Th e substitute answer must be the same as
the check answer if the substitute is correct. If our substitute answer
is diff erent, we know we have to go back and check our work to fi nd
the mistake.
I write the substitute numbers in pencil so I can erase them when I
have made the check. I write the substitute numbers either above or
below the original numbers, wherever I have room.
Th e example we have just done would look like this: