Answer : Casting out the nine is a method of checking answers to multiplication, addition and subtraction.
Casting out nines was known to the Roman bishop Hippolytos as early as the third century. It was employed by Twelfth-century Hindu mathematicians.
It is based on the idea of finding the sum of the digits in a number and then adding the digits in the resulting sum, etc., until a one digit number results. This addition of the digits in a number is further simplified by first discarding or casting away any digits whose sum is 9. The remainder is set down, in each case as the check number.
For example,
8216 x 4215 = 34630440, this is how you proceed.

4 + 2 + 1 + 5 = 12, casting out the nines leaves 3 as check number.

8 + 2 + 1 + 6 = 17, casting out the nines leaves 8 as check number.

3 + 4 + 6 + 3 + 0 + 4 + 4 + 0 = 24, casting the nines leaves 6 as check number.

3 x 8 = 24, 2 + 4 = 6 which was the check number of the original product. So the multiplication is correct.
Casting out nines is not in wide use today, mainly because of the widespread use of calculators and computers.