Multiplying Decimals 83
What reference number would we use for this one? We could use
10 for the reference number for 8, but 79 is closer to 100. Maybe
we could use 50. Th e speeds math method is easier to use when the
numbers are close together. So, how do we solve the problem? Why
not call the 8, 8.0?
Th ere is no diff erence between 8 and 8.0. Th e fi rst number equals
8; the second number equals 8 too, but it is accurate to one decimal
place. Th e value doesn’t change.
We can use 8.0 and work out the problem as if it were 80, as we did
above. We can now use a reference number of 100. Let’s see what
happens:
100 8.0 × 79 =
–20
–21
Now the problem is easy. Subtract diagonally.
79 – 20 = 59
Multiply 59 by the reference number (100) to get 5,900.
Multiply the numbers in the circles.
20 × 21 = 420
(To multiply by 20, we can multiply by 2 and then by 10.) Add the
result to the subtotal.
5,900 + 420 = 6,320
Th e completed problem would look like this:
100 8.0 × 79 = 5,900
–20
–21
- 420
6,320