3 Digit Addition Open Number Line Strategy: Numbers without Regrouping
Exposing students to many different strategies when learning how to add 3-digit numbers is helpful to reach all learners. The addition on an open number line strategy is helpful for students to keep track of useful parts of numbers when adding larger numbers.
Start with mastering addition using base ten blocks and addition 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 addition without regrouping using an open number line.
3 Digit Addition Open Number Line Strategy Example
What do you think you would do to start solving 231 + 337 using the open number line strategy?
You would start with an open number line, which is just a blank number line. You would decide which number to start on. In an addition problem, the starting number is your choice. It works both ways, so I tell students to pick one.
In this example, I chose to start on 337. I didn’t have a reason other than I like to start with the larger number on the number line. I placed 337 on the left side of the number line.
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, you look at the other addend to see what jumps of hundreds, tens and ones you can make.
In this example, I am adding 231 to 337.
I start making a jump of one hundred. When I add 100 to 337, the sum is 437.
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 337 + 231, I still need another jump of one hundred.
Once you finish adding all the hundreds, you are ready to add the tens.
Make another jump of 10.
Make one more jump of 10 to add 30 to 337.
You have now made two jumps of one hundred and three jumps of ten. You still need to add the 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 add using an open number line.
Your students might be able to make a jump of 20 instead of two jumps of 10.
As I mentioned earlier, you can start with any addend in an addition problem. So some students might start with 231 on their number line and make 337 jumps.
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 Addition Open Number Line Strategy Practice
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