# Insist on book algorithm?

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My DD is working on Singapore 2A, and is doing subtraction with regrouping.

Anyway, this morning, she went through her practice page at lightning speed-so fast that I suspected that she was working backwards by looking at the puzzle and figuring out what numbers had to be the answers.

When I quizzed her on it, she explained that she'd figured out how to not have to regroup by adding.

So, for a problem like

37-18, she'd add 2 to both numbers (mentally), which would give her 39-20, which she could then do mentally and get 19.

I did make her show me how to do the problems for 3 digit subtraction w/regrouping with counters and trading, and she understands the concept-but she very definitely was in the "let's humor silly mommy" mode, and simply couldn't see why we were doing it "the slow way".

So, when she does her practice page tomorrow, do I let her do it her way, or do I insist that she follow the book and do the borrow, carry, and so on?

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Since she can show that she knows re-grouping, I wouldn't worry about how she does it. I don't think there is anything at all wrong with her subtract-via-addition method (that's how I subtract myself!). It's quite efficient for larger numbers.

I'd just let her roll with it.

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Mental math works for us here! :001_smile: I would let her use novel ways to solve the math problems. Two of my children have always devised their own problem solving strategies. As long as they can explain the process to me, I let them alone. After all, the point is that they understand and accurately apply the concepts.

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she very definitely was in the "let's humor silly mommy" mode, and simply couldn't see why we were doing it "the slow way".

That's the beauty of homeschooling: you do not have to force her to do it the "slow way". Obviously she has a good grasp of the numbers and "sees" what is going on -so I don't see why she would have to do it some particular way.

(If I subtract in my head, I would do it the same way as she, btw)

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I would absolutely let her do it her way. It is great that she can find the way that works faster for her, and can explain it.

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My son does several types of math problems his own way. I teach it, he still does it whatever way he pleases. I have explained that his method might not always work, so he needs to know the other way too... but it's his brain and he can just do it that way (until he hits a snag).

It might also be that she sees the shortcut that most people don't catch on to until later.

Be forewarned, she will probably find other shortcuts in other ways.

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I afterschooled math for my older two when they were in middle school. I insisted they first learn and practice the problems/algorithm the "book" way. If a shortcut were found, then that could be used once I was certain the concept was learned properly.

It sounds like your dd learned has learned a mental math shortcut. I'd say let her use it when she can. If she should miss a problem using the mental math, then she'll have to show you the pencil and paper way :)

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