## New way of doing subtraction making its way around the net

This is real. If you’ve got kids, there’s a chance they might learn a completely different way of doing math than the way you were taught. Love it or hate it, there’s no question this is an interesting and logical way to solve a subtraction problem.

If you are struggling to wrap your head around this, think about handing someone a \$20 to pay for something that costs \$3.27 and watching them give you change. 3 cents to bring it up to \$3.30, than 70 cents to bring it up to \$4, then \$1 to bring it up to \$5, then another \$15 to bring it up to \$20. 3 cents plus 70 cents, plus \$1 + \$15 = \$16.73.

So 20 – 3.27 = 16.73, All done without borrowing.

UPDATE: To clarify, it’s certainly easy enough to use this “common core” approach when making change, but how would you solve the problem 96003.0023 – 9996.782? I can only do this using standard borrowing. For you folks who grew up with common core, how do you solve this problem?

• Suz

Learned how to do this ~20 years ago, selling flowers for an aunt of mine. It took a little rewiring to pick up, but then, SO easy. I love it!

• Saulk Pupét

I’ve done this all my life…I didn’t know it wasn’t how other people did it.

• Odi Kosmatos

“So 20 – 3.27 = 16.73, All done without borrowing.”

That’s because you handed them a \$20, but if you handed them a credit card you would definitely be borrowing. #rimshot

• http://dustinwilson.com/ dustinwilson

My father learned how to do this in elementary school, and he graduated high school in 1951. It’s hardly a new way of doing subtraction. He taught me how to when I was a kid.

The whole point of that method is that it’s really quick to do in your head, and you can use your knuckles and the spaces between them to do it rapidly with your fingers. It looks ridiculous on paper because it kind of is. The method existed in the old days because calculators didn’t exist. Today third graders get calculators and never retain how to do anything in their heads.

• Steve H

Sure, many of us have been doing this forever, but a lot of people haven’t. I’m dealing with common core for the first time this year with my 3rd grader. He’s decent at math, but he still struggles with doing some things in his head. The problem is exactly the same as described — remembering when you carried/borrowed. When I used the example in the linked article with him, he got it straight away. (And now I know what the homework sheets really want when they ask them to explain how they got the answer.)

• JohnDoey

This is not new. It’s funny that you used a cashier example, because cashiers have been doing this forever.

Making change from a \$5 or a \$100 is exactly the same using this method.

• Kelly Miller

Add 0.218 for the to get to 9997. Add 3 to get to 10000. Add 86003.0023 to get to the original number. So, 86006 + 0.218 + 0.0023 = 86006.2203.

The trick is to get to the power of ten, then add the extra to get to the original number.