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Lesson 5: Addition and subtraction within 100 word problems

# Subtraction word problem: crayons

Sal solves a subtraction word problem with numbers less than 100.

## Want to join the conversation?

• Can't it be 64 - 31 instead of 64 - _ =31?
• It means 64 minus what is 31, and it's just a different way of saying it.
• How do you know what number should go first when subtracting adding multiplying and dividing?
• When you are adding or multiplying, it doesn't matter which number goes first.
If you are not dealing with negative numbers, then you should be subtracting a smaller number from a larger one, same with division.
• -- Why is he able to move the number on the right just because he feels like it?

Oh, I guess I see it in pictures, but it's still confusing =( I'll try more.
• It is just a different way of writing the same equation. The rule with equations is that the value both sides of the '=' sign must always be equal even if we move the numbers around (that is why it is called an "equation"). As long as the equality still exists, it is OK to move the numbers in an equation around anywhere you like. In practice, this means if you move a positive number from one side of the equation to the other you must change it to make it negative, and if you move a negative number from one side of the equation you must make it positive.

In this case, as the unknown value represented by '?' is the value we want to find out, it makes sense to put it on one side of the equation (usually the left by convention) and all the other known values 64 and 31 on the other side. This makes it easier to work out the missing value on the left. Since we can now easily work out the value on the right side, as both sides of an equation must be equal, we will also know what the value is on the left side.

Hope that helps!