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I’ll deal with that number later

Today we are looking at a 2nd/3rd grade subtraction problem.

Let’s get started!

Quick question:  What is 120 really Do you know?  Can your child explain it to you?

Can they make that 120 friendlier?

I think that 100 + 20 is a lot friendlier than 120. It will be easier to take 38 away from 100, than it would have been to take it away from 120.

Since I don’t really need that 20 right now, I’m going to stick it on my forehead.

I’m holding that 20 in my brain and I’m going to come back to it later.

Sweet!  Now I can just deal with taking 38 away from 100.  That’s way easier.

100-38 on an abacus is super easy.  Don’t have an abacus?  Try a blank hundreds chart.

I like the Abacus because with enough practice kids start to visualize what 100 really is and know answers intuitively.

Ooops!  Is there still a number on my forehead?  I better deal with that 20 now.  62 + 20 = 82  I’m using adding (which I’m really good at) to help me do subtraction.

That means that 120 – 38 = 82!

But wait!  Did I really need a paper and pencil or could I have just done that problem in my head?  Hmm…

Math Boot Camp for Moms, Day 4


2 Comments

  1. Mrs. Warde says:

    These have been very interesting. I took a few college classes (a while ago) and the math professor had us using similar techniques. Even though it was a higher level math class, he put a big emphasis on mental math and practicing a lot of multi-level addition and subtraction. I am really grateful that he insisted on teaching us that, and it’s sad that I didn’t encounter it until college. I did 120-38 as 12-4 (120-40) and then added 2.

  2. Mrs. Warde says:

    (I meant 100’s and 1,000’s addition and subtraction. I don’t know where “multi-level” came from, but that’s not what I meant to put.)

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