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## Early math review

### Course: Early math review > Unit 2

Lesson 4: Put together, take apart# Addition and subtraction within 10

Sal solves addition and subtraction problems with number 10 or less.

## Want to join the conversation?

- So 1 + 4 is equal to 4 + 1 in0:45. Is this the commutative law?(18 votes)
- Yes, very good observation. It is also called the Commutative Property of Addition.(31 votes)

- why cant subtraction be backwards?(5 votes)
- Do you mean, why it is not commutative?

If you have 5 apples and someone takes away 2 of them, you are left with... 3. Am I right? 5 minus 2 is equal to 3 (5-2=3).

If you would like to change the order, you would get 2-5=? . What would it mean? You had 2 apples... and someone would take away 5 of THEM? it is impossible, it is higher mathematics called negative numbers which on this level is not necessary to know :). Hope that helps!(3 votes)

## Video transcript

- [Voiceover] So we're given
four different equations here and each of them have a blank, and we need to fill in that blank with a missing number. So let's do each of these. So on the left hand side of this equation, we have one plus four, so one plus four is the same thing as... Well, one plus four is five. If we take four, and if you add one more, you're gonna have five. So now, we finished that one, because one plus four is the same thing as five. Now here, we're saying four plus something is the same thing as five. Well, we just figured out that one plus four, or four plus one is the same thing as five. So four plus one is five. So notice there, we have the equation, four plus one is the same thing as five. Equals means that what's on the left is the same thing as what
you have on the right. Now we have blank plus two is equal to five. So what plus two is
the same thing as five? Well, three plus two is equal to five, or five is equal to three plus two. Now you go here. Well now you have three plus blank is equal to five. Well, we already figured it out, because three plus two is equal to five. So we can write three plus two is equal to five, and then we can check ourselves. And then we can keep going. So now we have some subtraction. So what's the same thing
as nine minus three? So if I have nine things, and if I were to take away three of them, I'm going to have six left. Nine minus three is the same thing as six. 10 minus blank is equal to six. So what do I have to take away from 10 in order to have six left over? Well, if I take four away from 10, I'm going to have six left over. Now something minus one is the same thing as six. Well, seven minus one is going to be equal to six. And then we get eight minus blank is equal to six. So how many do I have to take away from eight in order to get to six? Well, if I take away one, I'm going to get seven. If I take away two, I'm going to have six. So eight minus two is equal to six.