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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.
• 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.
• 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...
• 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.
• 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...
• Yeah, it's definitely a valid method for every propotion. Use it without any fear!
• does only practicing from khan academy enables to score well in SAT
• 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.
• 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...
• 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.
• 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.
• 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