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Home » Math » Don’t use an algorithm when mental math would be faster

Don’t use an algorithm when mental math would be faster

I’m your coach Jenny and I like to think about numbers.

No really, tell that to your kids. Clearly express to them that you are their coach and that you like math (whether that’s true or not).

Let’s start by warming up!

What’s wrong with this picture?

A third grader who draws all of those lines and junk on a simple problem like 1,000-344 = is telling you a lot. They are showing you that they are clinging to a traditional algorithm instead of using their brain.

We need to teach our kids to think.

There are LOTS of better ways to solve this problem. Today we are looking at one possible method.

I’m using number cards from my Right Start kit, but you could just use paper and pencil.

Notice that I’m laying out the problem in a horizontal fashion. This encourages children to let go of their algorithm life-raft and start using their brain instead.

Let’s start by thinking about the number 344. What do I need to do to 344 to make it a friendly number?

No wait! I’m going to make this even easier. I’m just going to think about the 44. What do I need to do to 44 to make it a friendly number?

I know. I could add 6 to it and get 50. 50 is a friendly number.

That means that 44 + 56 = 100. 100 is a really friendly number.

That makes me think that 344 + 56 must equal 400.

And you know what? I know that 600 + 400 = 1,000.

That means that 654 + 344 = 1,000. I’m using adding (which I’m really good at) to help me solve a big subtraction problem.

What do you know? I can do that problem in my head!!!

The real art in teaching math comes from asking questions.

How would you solve this problem? Do you have a way that you like better?  Feel free to leave a comment and tell us about your stratgey.  Or, you can just mock me for having numbers on my forehead.  Whatever…

Math Boot Camp will be back in session tomorrow!

Math Boot Camp for Moms, Day 1


  1. Crimson Wife says:

    The funny thing is that I’m currently fighting with my almost 7 y.o. to NOT use mental math because he’s working through a section in his Singapore math book where they are having the student practice the traditional algorithm. I finally had to pull a worksheet from my older child’s pre-algebra program where the task was to add three 5 digit numbers. The numbers were so big that my DS couldn’t just use the mental tricks but had to use pencil & paper.

  2. Amber says:

    I’m a high school math teacher and I think the “make it a friendly number” trick is great! I think the algorithm could be useful if students under stand what borrowing means (such as borrowing from the 10s column is like converting a solid stick of 10 counters into 10 individual ones) but students very rarely understand the meaning behind the algorithm. Anything with meaning is great! Making the math problems easy and adding to confidence is also excellent. Thanks!

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