I'm confused. Why don't you just multiply 9 percent by 1/3? Why do you have to add 1/3 to the product of 1/3 times 9? That is what confuses me. Is it because when you times 9 by 1/3 that only tells you how much the water expands? Then you have to add that amount to 1/3 to get the total amount of volume of water?

Yes, that's exactly it. Multiplying 1/3 by 9% will give you the amount of additioal volume that will be added when the water freezes, but to get the total volume of the ice, you'll have to add that addtional amount to the amount before freezing. The other way to do this is multiplying 1/3 by 109% (that's the same as multiplying it by 1.09).

Hi In this example sal took 9% of 1/3 and then added with original 1/3 = 109/300 = 0.3633 What would have happened if we evaluated 1/3 = 0.33 and then taken 9% of 0.33 = 0.0297 Hence final answer would have been 0.33 + 0.0297 = 0.3597. So can we say that taking % of fraction is more precise than converting it into decimal ? Thanks

Yes, usually working with fractions is more accurate than working with decimals, because when we use decimals we quite often create errors by rounding the numbers. You've given a good example, by saying that 1/3 = 0.33. It's not: 1/3 = 0.33333333... and so on forever. Try taking 9% of that instead -- you'll see that you get 0.03 and not 0.0297, so the final answer becomes 0.36333333..., and again so on forever. By contrast, the fraction 109/300 is totally accurate!

is it important that denominator should be positive, while comparing rational numbers

Good question. It makes it simpler to do the comparison if we don't have to worry about negative signs in the numerator and denominator. We can still compare them, but it's harder to do and we're more likely to make mistakes. Making sure that the denominator is positive is one of those maths rules that doesn't actually change the value of anything, it just makes it easier for us to use. It's like using capital letters at the beginning of sentences - that doesn't change the meaning of a sentence, it just helps us when we're reading by making the beginning of the sentence more obvious.

what is the definition of ratio

the quantitative relation between two amounts showing the number of times one value contains or is contained within the other.

Im confused why did he add? I thought he was going to multiply

you have to add the original amount you started with, plus what it expanded to get the total amount. To do this you take the total volume you started with plus 9% of that volume. Which gives you your total volume.

is thie sal khan (the owner) is he talking

Sal Khan is the founder and CEO of Khan Academy.

Main content

7th grade

Course: 7th grade > Unit 3

Lesson 4: Rational number word problems

Rational number word problem: ice

Name: Rational number word problem: ice
Uploaded: 2013-07-25T18:52:12Z
Description: Word problems force us to put concepts to work using real-world applications. In this example, determine the volume of frozen water and express the answer as a fraction.

CCSS.Math:

7.EE.B.3

Google Classroom

Word problems force us to put concepts to work using real-world applications. In this example, determine the volume of frozen water and express the answer as a fraction. Created by Sal Khan.

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Rashel
Posted 10 years ago. Direct link to Rashel's post “I'm confused. Why don't y...”
I'm confused. Why don't you just multiply 9 percent by 1/3? Why do you have to add 1/3 to the product of 1/3 times 9? That is what confuses me. Is it because when you times 9 by 1/3 that only tells you how much the water expands? Then you have to add that amount to 1/3 to get the total amount of volume of water?
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(21 votes)
Answer
- Gargarbl
  Posted 10 years ago. Direct link to Gargarbl's post “Yes, that's exactly it. M...”
  Yes, that's exactly it. Multiplying 1/3 by 9% will give you the amount of additioal volume that will be added when the water freezes, but to get the total volume of the ice, you'll have to add that addtional amount to the amount before freezing. The other way to do this is multiplying 1/3 by 109% (that's the same as multiplying it by 1.09).
  Comment on Gargarbl's post “Yes, that's exactly it. M...”
  (22 votes)
Harikesh
Posted 10 years ago. Direct link to Harikesh's post “Hi In this example sal to...”
Hi In this example sal took 9% of 1/3 and then added with original 1/3 = 109/300 = 0.3633
What would have happened if we evaluated 1/3 = 0.33 and then taken 9% of 0.33 = 0.0297
Hence final answer would have been 0.33 + 0.0297 = 0.3597.
So can we say that taking % of fraction is more precise than converting it into decimal ?

Thanks
Button navigates to signup pageComment on Harikesh's post “Hi In this example sal to...”
(13 votes)
Answer
- Ben Willetts
  Posted 10 years ago. Direct link to Ben Willetts's post “Yes, usually working with...”
  Yes, usually working with fractions is more accurate than working with decimals, because when we use decimals we quite often create errors by rounding the numbers. You've given a good example, by saying that 1/3 = 0.33. It's not: 1/3 = 0.33333333... and so on forever. Try taking 9% of that instead -- you'll see that you get 0.03 and not 0.0297, so the final answer becomes 0.36333333..., and again so on forever. By contrast, the fraction 109/300 is totally accurate!
  Comment on Ben Willetts's post “Yes, usually working with...”
  (9 votes)
harshvardhangalhotra
Posted 8 years ago. Direct link to harshvardhangalhotra's post “is it important that deno...”
is it important that denominator should be positive, while comparing rational numbers
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(3 votes)
Answer
- moomoosnake
  Posted 8 years ago. Direct link to moomoosnake's post “Good question. It makes i...”
  Good question. It makes it simpler to do the comparison if we don't have to worry about negative signs in the numerator and denominator. We can still compare them, but it's harder to do and we're more likely to make mistakes.
  
  Making sure that the denominator is positive is one of those maths rules that doesn't actually change the value of anything, it just makes it easier for us to use. It's like using capital letters at the beginning of sentences - that doesn't change the meaning of a sentence, it just helps us when we're reading by making the beginning of the sentence more obvious.
  Comment on moomoosnake's post “Good question. It makes i...”
  (6 votes)
californiahotdogsnl
Posted 7 years ago. Direct link to californiahotdogsnl's post “Why do you have to multip...”
Why do you have to multiply 1/3 into 1/3+9%
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(4 votes)
Answer
Talitha Hubbard
Posted 7 years ago. Direct link to Talitha Hubbard's post “what is the definition of...”
what is the definition of ratio
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(3 votes)
Answer
- Kaitlyn
  Posted 3 years ago. Direct link to Kaitlyn's post “the quantitative relation...”
  the quantitative relation between two amounts showing the number of times one value contains or is contained within the other.
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  (2 votes)
SV
Posted 8 years ago. Direct link to SV's post “is there a practice sessi...”
is there a practice session for this?
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(3 votes)
Answer
Dara Brenes
Posted 2 months ago. Direct link to Dara Brenes's post “I'm confused.... how did ...”
I'm confused.... how did he went from 1/3+3/100 to 100/300+9/300?
Button navigates to signup pageComment on Dara Brenes's post “I'm confused.... how did ...”
(2 votes)
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late melissa
Posted 7 years ago. Direct link to late melissa's post “Im confused why did he ad...”
Im confused why did he add? I thought he was going to multiply
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(1 vote)
Answer
- fr33d0g
  Posted 7 years ago. Direct link to fr33d0g's post “you have to add the origi...”
  you have to add the original amount you started with, plus what it expanded to get the total amount. To do this you take the total volume you started with plus 9% of that volume. Which gives you your total volume.
  Button navigates to signup page
  (3 votes)
lee
Posted 10 years ago. Direct link to lee's post “Why did he add 9%*1/3 to ...”
Why did he add 9%*1/3 to the original volume(1/3)? Is that how you find the volume...I forget. Sorry if this question is weird.
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(4 votes)
Answer
- Fieso Duck
  Posted 10 years ago. Direct link to Fieso Duck's post “Hi Lee, what the video ba...”
  Hi Lee, what the video basically says in the beginning is that if you have 1/3 gallon of water and you freeze it, the volume of this 1/3 gallon will increase by 9%.
  
  So after freezing your 1/3 gallon of water it's volume will have increased by 9%, so you are left with 100%*1/3 gallon + 9%*1/3 gallon, which is 109%*1/3 gallon.
  
  Later in the video this result is worked out as a fraction.
  Comment on Fieso Duck's post “Hi Lee, what the video ba...”
  (0 votes)
Dang_Mont
Posted a month ago. Direct link to Dang_Mont's post “is thie sal khan (the own...”
is thie sal khan (the owner) is he talking
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(1 vote)
Answer
- Joe H
  Posted a month ago. Direct link to Joe H's post “Sal Khan is the founder a...”
  Sal Khan is the founder and CEO of Khan Academy.
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  (2 votes)

Video transcript

Most liquids, when cooled, will simply shrink. Water, on the other hand, actually expands when it is frozen. Its volume will increase by about 9%. Suppose you have 1/3 of a gallon of water that gets frozen. What is the volume of the ice that you now have? So you're starting with 1/3 of a gallon of water. They tell us that when it gets frozen, when it turns into ice, its volume is going to expand by 9%. So the new volume is going to be your existing volume. So this is the original volume, 1/3 of a gallon, and it's going to expand by 9%. So your frozen volume is going to be your original volume plus 9% of your original volume. So you could say it's 9% times 1/3. So this right over here is going to be the expanded volume. Now, there's a bunch of ways we can figure it out. We could turn things to decimals or whatever else, but they tell us to express your answer as a fraction. So let's make sure that everything here is a fraction, and then we'll just try to simplify. So the one thing that's sitting here that is not a fraction is our 9%. Well, what does 9% actually represent? Well, 9% literally means 9 per 100. So we could rewrite this as-- so this is going to be equal to 1/3 plus, instead of writing 9%, I'll write that as 9 per 100, and then once again times 1/3. And we can simplify this expression right over here. We have a 9 in the numerator, a 3 in the denominator. If we divide both of them by 3, we get a 3 and a 1. And so we're left with 1/3 plus 300 times 1/1. Well, that's just going to be 3/100. So this is just going to be equal to 1/3 plus-- I'll write this in orange still, or maybe I'll do it in a new color-- plus 3/100. And now we have to add something, or two numbers that have different denominators. So let's find a common denominator. So this is going to be equal to, well, the least common multiple of 3 and 100. And they share no common factor, so it's really just going to be the product of 3 and 100-- the least common multiple is 300. So it's going to be something over 300 plus something over 300. Now to go from 3 to 300, in the denominator you multiply by 100, so you have to multiply the numerator by 100 as well. So 1/3 is the same thing as 100/300. And to go from 100 to 300, we have to multiply by 3 in the denominator, so we have to multiply by 3 in the numerator as well. So 3/100 is the same thing as 9/300. And now we're ready to add. This is going to be 100 plus 9/300, which is 109/300. So this is the volume of ice that I now have expressed as a fraction.