Saturday, March 10, 2012

Sinister Subtraction

I don't know about y'all, but in all my years of teaching I have yet to find something as difficult for students as subtraction. It is the one concept students will have one day and lose the next. In fact, sometimes they will work one problem correctly and when they get to the next one they completely lose it.

Yet, subtraction is one of the most important concepts students must master. Without it, figuring out change, calculating elapsed time, dividing and countless other skills are out the window. The one way to make sure that the concept of subtraction sticks is by making sure students understand the "Why?" of subtraction.

I am frequently asked by parents, "What happened to borrowing? Why is it called regrouping now?" Well, when you borrow something like a shirt that shirt doesn't change. It is still a shirt and eventually (hopefully) you return it. When you are "borrowing" with subtraction, you are never giving anything back to the tens or hundreds place where you took it from, and you are changing that ten or hundred into something else like ones. In other words, regrouping what you took into something totally different.

I spent many hours creating a PowerPoint presentation demonstrating the different ways students can subtract to solve problems. Some of these methods incorporate strategies from Singapore Math. Other methods have been gleamed from countless workshops and conferences. And some I found while scouring the Internet for ways to help make subtraction easier for students.

The four methods I teach students are:
1. Drawing (With this method, students do not even need to know their facts. It works really well with students who have a hard time remembering their basic math facts.)
2. Old School (The way we learned it, but with a few twists)
3. Take a Penny (This method has revolutionized my methods of subtracting across zeroes as well as my students.)
4. Fair is Fair (This works really well with 2 and 3 digit numbers and keeps students from having to regroup.)

Check out my Subtraction With and Without Regrouping at my TpT store. In the presentation, which I use over a 9 day period, I insert several videos that help illustrate the different methods.

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