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### Course: Digital SAT Math > Unit 3

Lesson 1: Ratios, rates, and proportions: foundations# Ratios, rates, and proportions — Basic example

Watch Sal work through a basic Ratios, rates, and proportions problem.

## Want to join the conversation?

- Couldn't Sal just have done 7 divided by 8 and then multiply the result, 0.875, by 20? That is how I did the problem in just seconds.(25 votes)
- He could have, but that is an advanced method and does not always work, and take note in mind, most people here are just starting to learn this and need to learn the basic method.(45 votes)

- Can't we cross multiply?

7/8=X/20

140=8X

X=140/8

X=17.5

I think this is simpler, but I'm not sure if this is a valid method for all problems...(26 votes)- You're right. Anytime you have two proportions set equal to each other, you can cross-multiply and it's perfectly valid. This is simpler than the method Sal chooses, but Sal's is easier to teach and explain the meaning behind, which is why I think he chose that way instead of cross-multiplying.(15 votes)

- Can't we cross multiply?

7/8=X/20

140=8X

X=140/8

X=17.5

I think this is simpler, but I'm not sure if this is a valid method for all problems...(13 votes)- Yeah, it's definitely a valid method for every propotion. Use it without any fear!(7 votes)

- does only practicing from khan academy enables to score well in SAT(10 votes)
- It depends on your math knowledge. If you are not used to these questions, you should use Khan Academy with another outside resource like IXL to practice Math questions. I believe that Khan Academy is very important, but if you really want to practice, I recommend doing many math drills to ace the SAT in the future.(4 votes)

- Can't we cross multiply?

7/8=X/20

140=8X

X=140/8

X=17.5

I think this is simpler, but I'm not sure if this is a valid method for all problems...(6 votes)- Yes, I do that a lot and - at least to me - it's definitely easier. Sal doesn't it like to do it because he likes to make sure you understand how it works, if you know what I mean. But, yeah, it's definitely a valid method.(8 votes)

- You guys should just use cross-multiplication in these questions. It is much simpler and also saves time as you gotta save as much time as possible.(8 votes)
- Please,Gift us more hard question(6 votes)
- It
*is*going to get harder and harder (I think)(1 vote)

- Can't we cross multiply?

7/8=X/20

140=8X

X=140/8

X=17.5(4 votes)- Yes, that is the best way to do it to avoid mental math errors. The way the video shows is easier when doing it without a calculator.(2 votes)

- I am so bad in math and I have psat tomorrow.(4 votes)
- Are all the questions shown on Khan Academy on the SAT?(3 votes)
- no. the SAT will have their own questions, but what's being taught here are what the questions are based off of(3 votes)

## Video transcript

- [Instructor] Seven pounds of plums make eight
rolls of fruit leather. If every batch of fruit leather
requires the same amount of plums, how many pounds
of plums are required to make 20 rolls of fruit leather? So, let's set up a ratio. We need seven pounds of plums for every eight rolls of fruit leather. Now, we need to think
about how many pounds of plums we're gonna need. How many pounds of plums we're gonna need to make 20 rolls of fruit leather, 20 rolls of fruit leather. Well, let's see, what have we done to go from eight rolls to 20 rolls? How much larger is 20 than eight? Let's see, 20 divided by eight is 2 4/8, which is 2 1/2. So we multiplied by 2 1/2 or we have 2 1/2 times as many rolls when we go from eight to 20. So we're gonna need 2
1/2 times as many pounds to keep the ratios constant. So times 2.5, and what's seven times 2.5? Let's see, seven times two is 14. Seven times five is 3.5. 14 plus 3.5, this is
going to be 17.5 pounds. Now, that's this choice right over here. And actually, even looking at the choices, you might have been able to get here even without doing this not
too intensive mathematics, because you can say, "Okay, look, "the pounds are kind of close
to the amount of leather, "but they're less than it." So we wouldn't want 23 pounds of plums if we make 20 rolls of fruit leather. This is more than 20. And if we're making more rolls than we're able to make
with the seven pounds, we're not gonna be able to
have fewer pounds there. So you could have actually
ruled out all of them just based on the logic to get to 17.5, but it's always satisfying
to do it the right way. But if you're under time pressure, deductive reasoning
isn't always a bad thing.