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### Course: Class 3>Unit 3

Lesson 3: Adding and subtracting missing value problems

# Missing number for 3-digit addition within 1000

Sal solves for the missing number in problems like ___ + 261 = 630.

## Want to join the conversation?

• How do you locate the difference of numbers in your head?
• @Aiden Delgado: Taking the difference of numbers in your head requires lots and lots of practice. You have to memorize arithmetic (e.g. 4-1 = 5, 8-6=2) until you recognize the differences when you see them. Keep working on your computer or on paper until you are able do the arithmetic in your head.
• I dont understand the subtraction part of finding the missing number with the number line.

For example- ?-435=210

The answer I got was 225. But checking a calculator it shows up that 225-435= -210

When I ask for a hint on the quiz, it tells me the answer is 645-435-210

Using a number line, how can I find out that 645 is the missing number for
?-435=210 ?
• I see what you did wrong. You subtracted 435 from 210, giving you a negative number: -225. That's not what you're supposed to do.

The problem is asking what number minus 435 will equal 210. So we automatically know the number has to be larger than 435. How larger? Well, it has to be 210 more than 435. If you're confused, consider the following, simpler equation:

? - 10 = 6

How would you solve something like that? You would add 10 to 6. That gives us 16. Our problem is true, because 16 - 10 = 6.

So do the same thing with our other problem. 435 + 210 = 645. Check with calculator, and we see that in fact 645 - 435 = 210.
• ? + 261 = 630 what is the question mark
• It's asking you what number plus 261 is equal to 630. The question mark is just standing for a number that we don't know yet.

To find out what it is... Remember that addition and subtraction are closely related to each other.
• Anyone have an easier way to explain this
• To do this more easier is that you can just subtract 261 from 630 which equals 369 so that means 369+261= 630. Hope this helps!. Bye! :)
• do you still subtract to find the answer when it is 557(blank) equals 106?):
• Yes. For 557 - blank = 106, you would do 557 - 106 = blank.
• how does it work like ?-3400=56
(its not a real question)
• I think you add 3400 and 56 to solve for "?".
• 𝓦𝓱𝓮𝓻𝓮 𝓭𝓲𝓭 𝓽𝓱𝓮 𝓓𝓪𝓿𝓲𝓭 𝓸𝓾𝓽 𝓰𝓸!??!??!??!??!??!??!??!??!??!??! _______________