ffirs.indd

192 Speed Math for Kids
numbers to solve problems in direct multiplication. If you try it and
you think it is too diffi cult, forget it—or come back to it in a year’s
time and try it again. I enjoy playing with this method. See what
you think. Here is how it works.
If you are multiplying a number by 79, it may be easier to use 80 – 1
as your multiplier. Multiplying by 79 means you are multiplying
by two high numbers and you are likely to have high subtotals.
Multiplication by 80 – 1 might be easier. Using 80 – 1, 8 becomes
the tens digit and –1 is the units digit. You need to be confi dent with
negative numbers to try this.
Let’s try it:
68 × 79 =
We set it up as:
6 8
8 –1
We begin by multiplying the units digits. Eight times minus 1 is
minus 8. We don’t write –8; we borrow 10 from the tens column
and write 2, which is left over when we minus the 8. We carry –1
(ten), which we borrowed, to the tens column.
Th e work looks like this:
6 8
8 –1
–1 2
Now we multiply crossways. Eight times 8 is 64, and 6 times –1
is –6.
64 – 6 = 58
We subtract the 1 that was carried (because it was –1) to get 57. We
write the 7 and carry the 5.