If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Lesson 3: Strategies for adding two and three-digit numbers

# Addition using groups of 10 and 100

Sal rewrites addition problems to make them easier to solve.

## Want to join the conversation?

• What is 555+795=? I don't really get it.

Write the two numbers one on top of the other, lining up the digits.

555
+ 795
------

Add the digits in the ones place (5 + 5 = 10). Write down the 0 and carry the 1 to the tens place.

555
+ 795
------
0

Add the digits in the tens place, including the carried digit (1 + 5 + 9 = 15). Write down the 5 and carry the 1 to the hundreds place.

555
+ 795
------
50

Add the digits in the hundreds place, including the carried digit (1 + 7 = 8).

555
+ 795
------
1350

So, 555 + 795 = 1350.
• How would this work when adding thousands?
• Similar to the first question in the video at , 632 + 4278 could be done like this:
(630+2)+4278
630+(2+4278)
630+4280

This equals 4,910. It's a bit more difficult in the thousands, but it can still be done in the same way.
• can you round 100?
• What did Sal mean when he said "a nice round number?"
• "Round" is sort of a term used to indicate it's ease of adding...so a number with a 0 at the end (10, 30, 110, 420, etc) are so easy to mentally add or subtract that they're considered "round" numbers. Think of it as "get around to an easier number". So if a number is 42, you "round" it down to 40, but if it's 46 you "round" it to 50, etc.
• 867 + 987 = what
• We can use a nice trick here! The trick comes from seeing that 987 is almost 1,000.

987 is just 13 less than 1000.
So we can add 867 + 1,000 to get 1,867, then subtract 13 to get 1,854 for the answer.

Have a good day!
• How do I know when I have to remove ones or tens ?
• It's really up to you. The point of removing "tens and ones" is to make it easier for you to do mental math. So do whatever is easiest for you.
• What is this property or rule called? Why is this allowed?
(1 vote)
• Associative law.
It states that (a + b) + c = a + (b + c)
• Can this same method be used on subtraction?
(1 vote)