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Lesson 4: Adding with regrouping within 1000

# Using place value to add 3-digit numbers: part 1

Let's explore adding two 3-digit numbers using place value and carrying. We walk through the process step by step, starting with the ones place, then the tens place, and finally the hundreds place. We emphasize the importance of understanding what carrying means and how it relates to place value. Created by Sal Khan.

## Want to join the conversation?

• why did he add a one above the nine?
• because you add it when it is over 10, so you add it to any number when you have 2 digits in your answer
• In adding why do you have to do it from right to left?Could we also do left to right?
• Actually, sometimes the numbers to the right go over 10. 144 + 379 does. You go from left to right so that if those numbers add up and have to go in over to a number to the left, you can do that without having to re-add that.

So basically, you could, but it would be very hard.
• Why do we have to do carrying?
• When adding 2 numbers, carrying is required when the numbers in the 1s place add up to 10 or more, or when the numbers in the 10s place add up to 100 or more, or when the numbers in the 100s place add up to 1000 or more, and so on.

If we want to add 98 and 3, we have a several steps to take:
`98 + 3 =`
`90 + 8 + 3 =` -- because `98 = 90 + 8`
`90 + 11 =` -- because `8 + 3 = 11`
`90 + 10 + 1 =` -- because `11 = 10 + 1`
`100 + 1 =` -- because `90 + 10 = 100` and this the same as our carry step
`101` -- our answer

This is a lot to do just to add 3 to 98, so we have this shortcut method called carrying to speed things up:

1) Add 8 and 3 to get 11. This gives us 1 in the 1s place and we carry 10 to the 10s place. So we add a 1 over the 10s place.
2) Add the carried 1 to 9 to get 10. This is equivalent to adding 10 to 90 to get 100. This gives us 0 in the 10s place and we carry 100 to the 100s place by adding a 1 over the 100s place.
3) Add the carried 100 to the 0 to give us 1 in the 100s place
4) We end up with 101

``    1   <- carried from 10 + 90     1  <- carried from 8 + 3     98   +  3    ---    101``
• so we carry because of if it were to be 14 if we added 127+347 we add 7+7 =14 so that 1 would go to the top of the other number then 4+2 then plus 1 correct?
• Correct. 7+7=14. The one would be added to the tens. Now you have 2+4+1=7. Then you add 1+3= 4. Your final answer would be 474.
• hello? Is any one here
• hello? Is any one here
(1 vote)
• so we carry because of if it were to be 14 if we added 127+347 we add 7+7 =14 so that 1 would go to the top of the other number then 4+2 then plus 1