If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

# 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.
(14 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.
(28 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...
(12 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.
(9 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...
(11 votes)
• Yeah, it's definitely a valid method for every propotion. Use it without any fear!
(3 votes)
• does only practicing from khan academy enables to score well in SAT
(9 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.
(2 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...
(5 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.
(7 votes)
• Please,Gift us more hard question
(5 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...
(4 votes)
• this is valid infact i dowith the samemethod and i get correct answer
(2 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.
(4 votes)
• Why didn't you cross multiply?
(2 votes)
• He could've done that, but he maybe thought that this way was easier and faster.
(3 votes)
• I am so bad in math and I have psat tomorrow.
(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.