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## Arithmetic (all content)

### Course: Arithmetic (all content)>Unit 3

Lesson 8: Relating multiplication and division

# Division word problem: school building

Sal uses a picture and understanding of multiplication to solve a division word problem.   Created by Sal Khan.

## Want to join the conversation?

• Can you divide anything by zero?
• No, we do not divide by zero.
Consider: suppose there were 9 cookies and you want to share them equally among 3 people, you divide them up so that each person get 3 cookies - right?
Now, suppose you want to divide the 9 cookies among zero people.
Can you see that dividing 9 cookies among zero people does not make sense?
How can it be done?
That is why dividing by 0 is not allowed - it makes no sense.
Mathematicians say that dividing by zero is "undefined".

These videos might interest you - though the math is more difficult than what you are doing right now:
• How meany numbers are there in the world??
• Numbers have no end as you can always add another number after one number. If you think 10000000 is it, it could also be 100000000
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• What does Sal mean when he said question mark is equal to 9?
• He illustrated the example using ? to represent the unknown number. When he reached the end of the problem, he found the unknown number was equal to 9 so ?=9.
(1 vote)
• But is each story the same? The bottom floor could be taller, at maybe 12 feet, making each other floor smaller. In addition, the top floor could be a storage room, and therefore be smaller.
(1 vote)
• It could be, but even if the top floor was not as tall as the others the answer would still be correct. You are asked to find the average floor height, not the actual floor height.
(1 vote)
• What do they mean by 7 stories?
(1 vote)
• They mean how many floors, like how houses have two or three floors.
(1 vote)
• How many windows do they have?
(1 vote)

Okay, so on three of the sides, the windows are seven by five, and seven times five is thirty-five.
Because there are three sides, you want to multiply 35 by three, and that is 105!

Now, the front is missing a window.

So 35 - 1 is 34.

105 plus 34 is 139.

So, this new KA building should have 139 windows. But, if there are windows on the doors, there are either 140 or 141, depending on if they have 1 or 2 doors.
(1 vote)
• From the problem statement, is it certain all the floors are the same height?
(1 vote)
• It is assumed they will be for the purposes of teaching averages, but in practice a building's floors can have different heights. Don't let that stop you from asking questions though!