Dear Carrie,

Check numbers are really quite fun. I think you and your child will enjoy learning about them and using them as an easy way to correct the computation of rows of numbers. It's a fast and efficient way to "check" if the basic math computation is correct. Another name for check numbers is "casting out 9's". Maybe you've heard of this? You add up the digits of a number and cast out the 9's and are left with a check number between 1 and 8. When you computer the check numbers they will figure out to have the correct check number of the final answer assuring you that you either got the correct answer or that you need to go back and check your computation. You will have a greater understanding of check numbers once you go through the three lessons that explain them in Level D.

Feel free to call, email or post any other questions you have re: check numbers or anything else related to RS.

Sincerely,
Rosine

Check Numbers are great fun, but somewhat confusing at first. Here is my step by step explanation with a problem illustrating it. Remember it is just another way to check to see if the problem is right. It also shows the interconnectedness in math. As your child sees this amazing pattern it will cause him to begin looking for other patterns and thinking more abstractly. It is worth the time and effort to teach. It works because our number system is a base 10 system. We only use ten basic digits and then repeat them to form the other numbers. Here goes with my first attempt at an explanation. Try it and if you are confused, give us a call at our customer service line 888-272-3291.

If I were adding: 2987 + 5496 my answer would be 8483. Let's pretend I got it wrong by forgetting to carry one nuumber and wrote : 8383. It is close enough so I may not catch the mistake, but with check numbers I will. We added vertically to get the answer, now we will add horizontally across the rows to check the answer. I cannot have two digits. If I get two I add them together again getting a final single digit. I take 2987 and add the digits to make: 26, I add them to make 8 which is my "check number" for this row. I could also cast out nines and I would get the same number. I remove any 9s or combination of numbers that equal 9. for instance, 2 & 7=9 so I cross them off, and I cross off 9 leaving only 8 [which is my check number for that row.] I do the same thing for the second row. 5496 added together equals 24, 2 + 4=6 which is my check number. Casting out nines in that row I would remove the 5 +4 because they equal 9, and the 9 leaving 6, which is the same check number from above. When I get those two check numbers I add them together to get the number I am looking for: 8+6=14, 1+4=5 When I add my answer together to check it, 8383 added together=22=4. I can see clearly that 5 is not 4 so I know I MADE A MISTAKE IN MY ADDING. That is the purpose of the check numbers, to check myself and prove my accuracy, which will be necessary in higher level math like algebra and geometry. If I had the right answer: 8483 added together it equals 23, which is 2+3=5! My answer matches my check numbers and life is swreet!!!

I hope this helps. The curriculum will explain it with pictures to make it more clear and tells you what you do with subtraction, but I hope this helps give you more confidence that you can do it. If you would like to practice with an adult who could walk you through it step by step on the phone our customer service line is 888-272-3291. Don't get overly stressed on these "nice to know" items. They are fun for the kids and they get them so much quicker than we do. Remember they are not having to relearn math like you are. Don't tell them it is hard and it won't be. It is just a fun way to add and to be able to know absolutely that your answer is right. The NCTM is big on having children prove their answers are right and this is one great way!
Happy learning!