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## Early math review

### Course: Early math review > Unit 2

Lesson 2: Making small numbers# Making 5

Learn how to add numbers to make the number five. Created by Sal Khan.

## Want to join the conversation?

- Are all negative numbers rational?(9 votes)
- Rational numbers are, by definition, any number that can be expressed as a fraction of whole numbers. Any decimal or non-decimal is rational. Positive numbers are rational, and so are negative numbers.(1 vote)

- How many times can you divide a fraction? I mean as in dividing the number 279/1908?(11 votes)
- You can divide a fraction an infinite amount of times, but it will get extremely small after you divide it about 5 or 10 times.(2 votes)

- just why does the number 5 so important?(4 votes)
- the number 5 is a physical representation of 5 things. although the symbol we use for 5 can be changed, the value of 5 will always exist because it is necessary for every number after 5(5 votes)

- what are rational and irrational numbers?(4 votes)
- A rational number can be expressed as a fraction, a irrational can't.Just a simple explanation, hope this helps!(6 votes)

- 353,345,897 please order(4 votes)
- 345, 353, 897 You do this by looking at the first place values which are 3,3, and 8. So you can tell just by the 8 that it will be last. Let us go back to the 3's. Since they both have 3 in the first place value move to the second. In the second place value, they have a 5 and a 4. So 4 is LESS than 5. So the one with 4 goes first. The one with 5 goes second, and the one that has 8 goes last :)! This is called least to greatest. If you wanted greatest to least it is 897, 353,345!(5 votes)

- How do you know all the different ways to make five. by LilyAnna(5 votes)
- Technically speaking there are infinite ways to make five. An example would be 10,000 - 9,995 = 5 or -5 + 10 = 5 and you could go on from there.(3 votes)

- so you wont me to cont by 5's(3 votes)
- we should make this like the most upvoted post so they actually come back(2 votes)

- Isn't there like no difference between making 5, 10, and 20(0 votes)
- Yes, their is a difference between those numbers.(1 vote)

- comeon,how many ways to make fives(3 votes)
- 5 is an odd number. So there will be
**1 way**, 5 is a prime**number**.(0 votes)

- who is the man in the video(3 votes)
- Sal Khan, I think like the creator of Khan Academy.(1 vote)

## Video transcript

Voiceover: What I want to do in this video is think about the
different ways to make five. What do I mean about that? Right over here we have
three plus something, so there's some unknown
number right over here. Three plus something is
going to be equal to five. In this diagram we have three circles here that represents the three and then we have five boxes. One, two, three, four, five. How many more circles do
we need in order to fill up all the five boxes? We've clearly already filled three. Well, let's think about it. We just have to add one and two circles. Now we have a total of five. Three blue ones and two
of this purple color or I guess this pink color. We see that three plus
two is equal to five. That mystery number
here, that blank is two. Three plus two is equal to five. Now over here we have two plus a mystery number
is equal to five again. Once again, we have five spaces and two of them are filled in with this kind of purple, pink stars. How many more do we have to fill in in order to fill in all five boxes, two plus what is equal to five. I encourage you to pause the video and think about it. I'm assuming you've had a go at it. Let's think about how
many more of this boxes we need to fill in, in order
to fill in all five of them. We need to fill in one,
two, and three of the boxes in order to fill out all five. Now we have five stars. We have two of the pink ones and we have three of the blue ones. We can say that two plus three, the mystery number here is three. Two plus three is equal to five. Let's do a few more of this. Here I have four plus some mystery number is equal to five. I filled out four of the five boxes, how many more do I need to fill in in order to have five things, in order to have all
five boxes filled out. Well we see very clearly here that there is exactly one box that we need to fill in or one box that we need
to add to the other four. We see that four plus
one is equal to five. Now let's go the other way. We have the five boxes, only
one of them is filled in. How many more of these smiley faces do we need to add, in order
to fill in all the boxes? Or another way of thinking about it, we have one, how many more do we need to add to one to get to five? Like always, I encourage
you to pause the video and think about it on your own. Well, let's see, we can add one, we can add two, we can add three, and four yellow smiley faces. We could say one, one we
have one white smiley face plus one, two, three,
four yellow smiley faces is equal to five. You see a pattern here. Let's start at the beginning. Three plus two is equal to five, and two plus three is equal to five. When you had three filled in, you needed two more to get to five. When you had two filled in, you needed three more to get to five. Notice three plus two, two plus three. Same thing over here, four plus one is equal to five, one plus four is equal to five.