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. This is the same as our check answer, so our original answer is correct.
Had we got an answer that added to, say, 2 or 5, we would know we had made a mistake. The
substitute answer must be the same as the check answer if the substitute is correct. If our substitute
answer is different we know we have to go back and check our work to find 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.
The example we have just done would look like this:
If we have the right answer in our calculation, the digits in the original answer should add up to
the same as the digits in our check answer.
Let’s try it again, this time using 14 × 14:
14 × 14 = 196
1 + 4 = 5 (substitute for 14)
1 + 4 = 5 (substitute for 14 again)
So our substitute numbers are 5 and 5. Our next step is to multiply these:
5 × 5 = 25
Twenty-five is a two-digit number so we add its digits:
2 + 5 = 7
Seven is our check answer.
Now, to find out if we have the correct answer, we add the digits in our original answer, 196:
1 + 9 + 6 = 16
To bring 16 to a one-digit number:
1 + 6 = 7
Seven is what we got for our check answer so we can be confident we didn’t make a mistake.