Hello Brenda,

Dr Cotter will be answering your question next week, so please be patient, as your son's question is a good one and he may enjoy the answer.

Thank you,
Carissa
Customer Service Rep

Hello Brenda,

Here's how check numbers, also called "casting out nines," work. A check number, is the remainder after dividing by 9. At a simpler level you might think of breaking each number into piles of 9s with the leftovers kept in a separate pile. The leftovers are the check numbers.


For example, add 21 + 13. The 21 will have 2 piles of nines and 3 leftover; the 13 will have 1 pile of nine and 4 leftover. The sum is 34, which has 3 piles of nines and 7 leftover. We only check leftovers to be sure they match. In this case (3) + (4) equal the (7).


Actually, you could use any number, not just 9s for check numbers. Think about using 5s. Then the check number for 12 would be 2 and the check number for 16 would be 1. Add them together, 12 + 16 = 28 with a 5s check number of 3. It is not very useful because the only errors you'd find would be in the ones place.


We use nines because it finds the most errors and the check numbers are fairly easy to calculate.


Thank you,
Joan A. Cotter

Thank you for the clear explanation, Dr. Cotter. We appreciate your time!

We also appreciate these math books, challenging though they may be at times. They do help in understanding math concepts.