- Your check could look like this:
Note that this only works with addition. With all other mathematical checks you must find substitutes
for each number. With addition you can cross out (cast out) any digits of the numbers you are adding.
But remember, you can’t mix digits in the numbers you are adding with digits in the answer.
Let’s look at another example.
Here the 2 in 234 can be crossed with the 7 in 671. The 3 in 234 combines with the 6 in 671. The 4 in
234 combines with a 5 in 855. The 1 in 671 combines with the 8 in 855. All digits are cast out except a
5 in 855. That means the answer must add to 5. Let’s now check the answer:
1 + 7 + 6 + 0 = 14
1 + 4 = 5
Our calculation is correct.
The final check looks like this:
If the above figures were amounts of money with a decimal, it would make no difference. You can
use this method to check almost all of your additions, subtractions, multiplications and divisions. And
you can have fun doing it.
Note to parents and teachers
The basic number facts are quickly learnt when children master these techniques and learn the multiplication tables.
Casting out nines to check answers will give plenty of practice with the combinations of numbers that add to 9. These exercises
don’t need to be drilled. They are learned naturally when students use these strategies.
Once a student knows the basic make-up of numbers, he or she can easily add small numbers without mistake and without
borrowing.