We are working through Level B on mentally adding with 2-digit numbers. We do pretty good with these:
30+7,35+60,68+4, 42+80, 21+15, etc. With those types of equations they use their 5's strategy, 9's trick, and 8's trick well. However, we fumble around quite a bit when we get to these from Lesson 76: 36+56, 58+59, 54+57
I am afraid I am teaching them the wrong approach, and I do not want to venture off the Right Start rails. Am I wrong in saying to them: "36+56...there are 3 tens and 5 tens so we have 8 tens or 80. And we have 6 ones and 6 ones (which can be added as a double, of course, or using the 5's strategy) which is 12. Therefore, we know how 80 and 12. So we get 92."....is our approach to that correct? It seems like the book says I should do it this way: "36 and 30 is 66. So then we are left with 66 and 6. Then we will have 60 and 12 (since we will need to add 6 and 6)....
Is one way a wrong "thinking" approach? I don't want to mess up the Right Start process.

Thank you!

Melissa,

Good question! I went to Dr. Cotter and asked her which was best and why.

Let's look at a written problem: 28 + 59. Both you and the child can see the numbers, so there is no chance of "forgetting" the problem. Adding 2-ten and 5-ten gives 7-ten, or 70. 8 and 9 is 17. Adding 70 and 17 gives an answer of 87.

This is what the RightStart method tells us to do and this is exactly how I do it. But, if I'm doing it orally, I have to keep 28, 59, 70, and 8, 9, 17, then 70 and 17 all in my memory to come up with 87. Personally, I find that challenging some days!

Let's assume the same problem, 28 + 59, is oral. (No more peeking back to see what the numbers were again). Feeling a bit panicky? I am! "28, 59, 70, 8, 9, 17, 70, 17, 87." Whew.

So, let's try a slightly easier path to keep the numbers in our short term memory. To clear our minds, let me restate the problem: 28 + 59. Add '2-ten 8' and '5-ten' is '7-ten 8' and 9 more is '8-ten 7', or 87. Using traditional numbers, it sounds like "28, 59, 78, 9, 87".

We don't have to remember as many numbers when applying this method, therefore less likely to forget something (which is generally the ones). This second method is common in the Netherlands - it's not just a RS approach!

To answer your question, "is one way a wrong thinking approach?" No. It just that one method has benefits over the other and is more applicable in particular situations. Personally, I tend to use the first method, but then, I will confess to asking "what was the numbers again?" because I cannot keep them all in my head! I guess I'll have to give the second option a try and see if I find it more applicable for oral situations.

Which is best? It depends on whether it's oral or written. Both are correct! And both are better than scrambling for a paper and pencil!

I hope this helps. I realize I'm rather long winded, but you have a great question.

Kathleen