Because using a minus substitute changes the sign — that is, a plus changes to minus and a minus
changes to plus — −8 has a minus substitute of + 1.
Our check now becomes 3 + 1 = 4, which we see is correct.
Of course, we wouldn’t normally use this check for a problem like 12 – 8, but it is an option when we
are working with larger numbers. For instance, let’s check 265 — 143 = 122.
We find the substitutes by adding the digits:
Our substitute calculation becomes 4 − 8 = 5.
If we find the minus equivalent of−8 we get + 1. So, our check becomes 4 + 1 = 5.
We can easily see now that our answer is correct.
Even though you might not use negative substitute numbers very often, they are still fun to play with,
and, as you use them, you will become more familiar with positive and negative numbers. You also
have another option for checking answers.