ffirs.indd

172 Speed Math for Kids
subtracting from. Th is is because minus substitutes change a plus to
a minus and a minus to a plus.
How does this help? Let’s see.
If we want to check 12 – 8 = 4, our normal substitute numbers
aren’t much help.
12 – 8 = 4
3 8 4
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 fi nd the substitutes by adding the digits:
265 – 143 = 122
13 8 5
4
Our substitute calculation becomes 4 – 8 = 5.
If we fi nd 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.
cc17.indd Sec1:172 17 .indd Sec 1 : 172 1/9/07 8:40:53 AM 1 / 9 / 07 8 : 40 : 53 AM