Another way to do subtraction

No more borrowing and paying back, or the new alternative

Use addition and then simple subtraction, naturally there are several steps

Example     234 – 187


-187  (should be lined up!)

Now 7 is greater than 4,  so we add 3 to both numbers, giving a zero in the units position of the second number

234 -187 = 237 – 190   (must be able to add 3 to 187 !!!)

moving to the 10’s position, and very formally, 90 is greater than 37 so add 10 to both numbers, giving

237 – 187 = 247 – 200, which is now simple, with result  47

Of course, if the lower digit is less than the one above just subtract it from each number

234 – 117 = 237 – 120 = 214 – 100 = 114

It is best to keep the “one above the other” layout, but I have fought with the editor enough today.

1 Comment

Filed under arithmetic

One response to “Another way to do subtraction

  1. I am following you now. This is a version of equivalence statements or sentences, something I was going to blog about later this summer. Equivalence statements are a great way to expand student’s thinking and get them to formulate faster mental calculations. Thanks for the post!

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