ffirs.indd

Checking Your Answers 39
calculators can’t handle the number of digits in the answer, so most
would show the fi rst digits of the answer with an error sign.
Th e easy way to check the answer is to cast out the nines. Let’s try
it.
12,345,678 0
89,045 = 8
1,099,320,897,510 0
All of the digits in the answer cancel. Th e nines automatically cancel,
then we have 1 + 8, 2 + 7, then 3 + 5 + 1 = 9, which cancels again.
And 0 × 8 = 0, so our answer seems to be correct.
Let’s try it again.
137 × 456 = 62,472
To fi nd our substitute for 137:
1 + 3 + 7 = 11
1 + 1 = 2
Th ere were no shortcuts with the fi rst number. Two is our substitute
for 137.
To fi nd our substitute for 456:
4 + 5 + 6 =
We immediately see that 4 + 5 = 9, so we cross out the 4 and the 5.
Th at just leaves us with 6, our substitute for 456.
Can we fi nd any nines, or digits adding up to 9, in the answer? Yes,
7 + 2 = 9, so we cross out the 7 and the 2. We add the other digits:
6 + 2 + 4 = 12
1 + 2 = 3
Th ree is our substitute answer.