ffirs.indd

38 Speed Math for Kids
+4
+4
10 14 × 14 = 180
5 5 + 16
196
7
A shortcut
Th ere is another shortcut to this procedure. If we fi nd a 9 anywhere
in the calculation, we cross it out. Th is is called casting out nines.
You can see with this example how this removes a step from our
calculations without aff ecting the result. With the last answer, 196,
instead of adding 1 + 9 + 6, which equals 16, and then adding 1 +
6, which equals 7, we could cross out the 9 and just add 1 and 6,
which also equals 7. Th is makes no diff erence to the answer, but it
saves some time and eff ort, and I am in favor of anything that saves
time and eff ort.
What about the answer to the fi rst problem we solved, 168? Can we
use this shortcut? Th ere isn’t a 9 in 168.
We added 1 + 6 + 8 to get 15, then added 1 + 5 to get our fi nal check
answer of 6. In 168, we have two digits that add up to 9, the 1 and
the 8. Cross them out and you just have the 6 left. No more work to
do at all, so our shortcut works.
Check any size number
What makes this method so easy to use is that it changes any size
number into a single-digit number. You can check calculations that
are too big to go into your calculator by casting out nines.
For instance, if we wanted to check 12,345,678 × 89,045 =
1,099,320,897,510, we would have a problem because most