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## Arithmetic (all content)

### Course: Arithmetic (all content)>Unit 2

Lesson 1: Basic addition and subtraction

# Add and subtract: pieces of fruit

Sal adds fruit and subtracts fruit. The examples he uses are 5+3 and 5-3. Created by Sal Khan.

## Want to join the conversation?

• When you get to large numbers such as trillion and quadrillion... are there any real world applications for such large numbers? Is there a quadrillion or quintillion of anything in the universe?
• For example, the mass of the Earth is about 6 million times as large as a quintillion kilograms (that is, the mass is 6,000,000,000,000,000,000,000,000 kilograms).

Another example: the number of molecules in an average drop of water (0.05 millilters) is more than 1,600 quintillion (that is, more than 1,600,000,000,000,000,000,000 molecules).
• Are there more cherries or more blueberries?
• There are `5` blueberries, which is more than the `3` cherries. In fact, in the video he subtracts `5 - 3` to show what happens if you start with `5` blueberries and then eat `3`. But you can also use `5 - 3 = 2` to find how many more blueberries there are than cherries. He has `2` more blueberries than cherries.
• I just realized that whenever 5 is first, if you both add and subtract the same number from 5, the two results will always add up to 10. For example
5+1 = 6
5-1 = 4 => 6+4 = 10
Even with numbers larger than 5
5+7 = 12
5-7 = -2
12 + (-2) = 12-2 = 10
And it will go on, 13-3, 14-4, 15-5, you see the pattern? The last digit is always the same.

Does this thing have a name?
Oh wait, how could I be so stupid, it is simple math, if (5+1)+(5-1)=
5+1+5-1=5+5=10. Duh, 1 and -1 neutralize each other.

Anyway, does this have a name?
• I don't think it has a name. You are basically adding the same number, because you cancel out the other number. So it always will end with an addition of 2 equal numbers: 2+2, 4+4, 7+7, 12+12, etc...
(1 vote)
• If there was 1 hundred pieces of fruit, and you wanted to put them into 20 bags evenly, what would I do?
(1 vote)
• You'd put five fruits into each bag. That is, unless you are including the difference of mass in each fruit; in that case I couldn't help you....
(1 vote)
• So what happened if it is 2-3, how would you demonstrate using the above example?
(1 vote)
• So, I'm going on a stretch here, hoping that you understand the idea of owing someone something.

Imagine that I come to your fruit shop, and pay money for 3 apples. But it's kind of late in the day, and the shopkeeper (you) only have 2 apples left. I say, "That's okay! Just give it to me when you get another apple!"
So then, you owe me an apple. Whenever you get another stock of apples, you will have one less, because you owe it to me.

In mathematical terms, I would represent it like this:
2 - 3 = -1
OR
= 2 - 2 - 1
= 0 - 1
= -1

In imagery:
🍎🍎 - 🍎🍎🍎 = -🍎

For the ideal KA explanation, check out Sal's explanation:
(1 vote)
• What happens if you repeat addition?
• In higher mathematics, you learn about *multiplication,which I also known as repeated addition*.

For example, you can do 3*2.
3*2 = 3+3
OR
3*2 = 2+2+2
Either way you do this, you end up with the same answer: 6 (because 2+2+2 = 6 and 3+3 = 6.) Therefore, 3*2 = 6.

I hope this helped you to understand repeated addition!