How to Teach Decimal Addition and Subtraction
How do you teach adding and subtracting decimals in upper elementary or middle school math?
In 6th grade, my math students have typically come to me knowing the 'rules' for adding and subtracting decimals.
However, when the number of digits in the numbers they're adding or subtracting aren't the same, they don't necessarily line the numbers up the way they need to...even though they 'know' the rules. Why is this?
I believe it's because they really don't understand the point of 'lining up the decimal points.'
My belief is reinforced by student comments I collected one year as we began our decimal operations unit.
I asked my 6th grade math students to solve 35.2 + 7.489 and then explain why their answer made sense. These are a few of their responses:
Of the 120 students in my classes, only 8 said the answer made sense because "35 + 7 is 42" or because "I estimated" or "when we're doing addition, we know we end up with a bigger number."
I don't want to assume that students who didn't write this didn't think about those things at all, but to the majority of students, their answers "made sense" when they followed the rules - even if they didn't remember the rules correctly.
What's the Point?
What's the purpose of lining up the decimal points?
Lining up the decimal points helps us line up the place values so that place values are added with or subtracted from 'like' place values.
If students don't understand the point of lining up the decimal points when adding and subtracting decimals, then somehow they've missed the idea of place value.
And what do we do if they numbers don't even HAVE a decimal point?? (Some students get pretty lost when that happens.)
Start With Estimating When Adding or Subtracting Decimals
Because I believe students don't understand the point of lining up the decimal point, I teach them to add and subtract decimals by doing the following:
Tools for Teaching Decimal Addition and Subtraction
One of my favorite ways to teach the addition and subtraction process is to use notes that emphasize estimating.
I include this in my unit notes and practice, and I also use math wheels for note-taking.
The math wheels break the processes into steps and allow room for examples and practice. They also give students a chance to color-code, doodle, color, and add memory triggers to their notes. Then they can keep them in the notebooks all year for reference.
Adding Decimals Wheel
Subtracting Decimals Wheel
A couple practice decimal activities:
There are a couple free decimal activities found in the blog posts linked below that I've used to give my students some additional decimal operations practice.
Decimal Operations Problem Solving
Decimal Practice with Number Puzzles
What are your tried and true methods for teaching adding and subtracting decimals?
Resources to Practice Adding and Subtracting Decimals:
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Hey there! I'm Ellie - here to share math fun, best practices, and engaging, challenging, easy-prep activities ideas!