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Course: Early math review>Unit 6

Lesson 4: Strategies for adding 2- and 3-digit numbers

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?
• 54-65 would create a negative number

50-60 = -10
4-5 = -1
-10 + -1 = -11
• What does 256 round to the nearest 10?
• 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.

Have a blessed, wonderful day!
• Why use number lines when you can use the standard algorithm because number lines take to much time
• 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.
• 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)