3 Digit Subtraction Open Number Line Strategy
Exposing students to many different strategies when learning how to subtract 3-digit numbers is helpful to reach all learners. The subtraction open number line strategy is helpful for students to keep track of useful parts of numbers when subtracting larger numbers.
Start with mastering subtraction using base ten blocks and subtraction using break apart. Once students master those strategies, you can move on to the open number line strategy. Each strategy builds and requires students to move from concrete thinking to abstract thinking.
The standard 2.NBT.7 uses many different strategies to master 3-digit addition and subtraction. I take it very slowly to help with mastery of this standard. I give students many chances to find a strategy that works for them before teaching the traditional algorithm.
Keep reading to learn more about the open number line strategy and to find a resource you can use right in your classroom to teach 3-digit subtraction without regrouping using an open number line.
3 Digit Subtraction Open Number Line Strategy Example
What do you think you would do to start solving 578 – 212 using the open number line strategy?
You would start with an open number line, which is just a blank number line. You wouldn’t need to decide which number to start on, like you do in an addition problem. In a subtraction problem, the starting number is the minuend (578 in this example).
Place 578 on the right side of the number line. (This is different from addition, where you start on the left side.)
Many students will want to start drawing tick marks to keep track of numbers, but that ends up being a waste of time.
Once you have your starting number on the number line, you look at the subtrahend to see what jumps of hundreds, tens and ones you can make.
In this example, I am subtracting 212 from 578.
I start making one jump of one hundred. When I subtract 100 from 578, the difference is 568.
Note: It is very important that your students write the sum after their jumps as they go. It is easy to make a mistake if you make all the jumps and fill in the sums after.
Since the problem is 578 – 212, I still need one more jump of one hundred.
Once you finish subtracting all the hundreds, you are ready to subtract the tens.
You have now made two jumps of one hundred and one jump of ten.
You still have to subtract the ones.
You have now subtracted the hundreds, tens, and ones.
To help my students remember to write the sum when they make their jumps, I have them think of an airplane taking off from one airport and landing at another airport that many jumps ahead. They have to know where the airplane lands before it can take off for another airport.
An open number line can be used in many different ways. The example I showed was just one way to subtract using an open number line.
Your students might be able to make a jump of 200 instead of jumps of 100.
I make sure students understand jumps of hundreds, tens and ones before showing them how to make jumps with larger numbers.
An open number line provides many options for differentiation within the same strategy.
3 Digit Subtraction Open Number Line Strategy Practice
Do you need some practice pages for your students to use with this strategy?
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