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### Course: Early math review > Unit 6

Lesson 4: Strategies for adding 2- and 3-digit numbers- Breaking apart 3-digit addition problems
- Break apart 3-digit addition problems
- Solving 3-digit addition in your head
- Addition using groups of 10 and 100
- Add using groups of 10 and 100
- Adding and subtracting on number line
- Add on a number line
- Select strategies for adding within 1000

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# Adding and subtracting on number line

Sal adds and subtracts numbers like 585 and 368 using a number line. Numbers used in these problems are less than 1000.

## Want to join the conversation?

- Can you subtract 54-65? Can you? If you can, what is the answer?(31 votes)
- 54-65 would create a negative number

50-60 = -10

4-5 = -1

-10 + -1 = -11(37 votes)

- What does 256 round to the nearest 10?(4 votes)
- Look at the digit just to the right of the tens digit, that is, the units digit. This digit is 6.

6 is 5 or greater, so round up. Add 1 to the tens digit. Change all digits to the right of the tens digit to 0's.

The answer is 260.

Have a blessed, wonderful day!(24 votes)

- Why use number lines when you can use the standard algorithm because number lines take to much time(0 votes)
- Number lines are a way to add and subtract number intuitively.

Of course you can use your "standard algorithm", but the number line helps people who don't understand this topic to visualize it.(8 votes)

- Is there a way to add or subtract on even 2 number line(s)?(1 vote)
- There is a way to use two rulers for addition.

Let's say we want to do 7 + 2.

Place the graded sides of the rulers against each other so that the 7 on one ruler matches up with the 2 on the other ruler.

This way the 0 on either ruler matches up with 9 on the other ruler.

Therefore, 7 + 2 = 9.

This is the same thing as reversing the order of one number line

and placing them so that 7 on one number line matches up with 2 on the other.

Then 0 on either number line matches up with 9 on the other.

Subtracting a positive number is the same as adding a negative,

so to do 7 − 2, we match up 7 and −2.

Then 0 will match up with 5.

So, 7 − 2 = 7 + (−2) = 5.(1 vote)

- Thanks for the tips! ;3(1 vote)
- I'm having trouble to communicate with the numbers. I need a guide from some of you!(1 vote)
- Sure it just takes so much practice(1 vote)

## Video transcript

- Which number line shows 361 + 544? Let's see, in all of them we're starting at 361, so now let's add 544. This one starts with adding 400, and then 50, and then 4, it's adding 454, not 544. Now this one adds 500, then 40, and then 4, so this is adding 544, to 361, we see that it is 905. This makes sense, you add 500, you get 861, you add 40, you get to 901, and then add the 4, you get 905, so definitely go with that choice. This one over here, instead of add 500, you add 50, which doesn't make sense because we're clearly adding 5 hundreds, 4 tens, or 40, and then 4 ones, let's do some more examples. Which expression can be solved with the following number line? Let's see, we're starting at 718, and then we're adding 200, and then we're adding 40 to get to 958. This is really 718 + 240, which is this choice right over there. This is kind of, strangely fun. Which number line shows 585 - 368? Let's see, these second two number lines, both start at 585, we're starting our computation at 585, this one's starting at 558, so it swapped the numbers, so this one doesn't make sense, but let's look at these two choices. We want to subtract 3 hundreds, 6 tens, and 8 ones. In both of these, we subtract 3 hundreds, and then we want to subtract 6 tens, 6 tens, and then 8 ones, that's this one over here. This one swaps the... instead of having 6 tens and 8 ones, it has 8 tens and 6 ones, so it swapped... this is subtracting 386, which isn't the original problem, so definitely like this third choice. Let's do one more example. Which expression has been solved with the following number line? We're starting at 935, and then we're going to subtract 400, and then we're going to subtract 20. This is 935 - 420. 935 - 420, that choice right over there.