Lesson: Ratios and Rates

Comment on Ratios and Rates

Hi Brent,

At 3.20,

We had an equation

70/3.5 = T/60

So, I simplified before cross multiplying, which led me to
cancelling the zeros in 70 and 60.

So it came to 42/3.5 , which is equal to 6/0.5.

What was the mistake in my approach?
greenlight-admin's picture

You're taking a strategy used solely for MULTIPLYING FRACTIONS, and applying it to EQUATION SOLVING.
For example, the PRODUCT 4/11 x 7/8, can be cross simplified to get 1/11 x 7/2, which equals 7/22

This technique can't be applied to an EQUATION.
For example, if we take the TRUE equation 20/40 = 10/20 and try to cross simplify in the same manner, we get 1/4 = 1/1, which is NOT TRUE.

Hi Brent!
In the 150-159 difficulty question, I am super confused on the following:

I understand how you got Machine II makes 48 copies in 1 minute. But then, should you not multiply that by 6 to get the 6 minute output? Why did you quadruple?
greenlight-admin's picture

Question link: https://gre.myprepclub.com/forum/working-at-their-respective-constant-ra...
The question asks us to determine the number of copies machine II makes in 4 minutes (not 6 minutes).


I solved this problem as follows 2x+4/5x = 3/5 solve for x we get 4, is the question asking about the total workers including the 4? If i did that I get 32 or just the total workers (2*4+5*4), are including the 4 or not was not clear?
greenlight-admin's picture

Question link: https://gmatclub.com/forum/in-a-certain-store-the-ratio-of-part-time-wor...

Good question!
We need to follow the verb tenses here.

"In a certain store, the ratio of part-time workers to full-time workers is 2 to 5." This is the PRESENT situation.

"If 4 part-time workers were hired, then the ratio would be 3 to 5." This is a HYPOTHETICAL situation.

"How many workers DOES the store have?"
"DOES" means we're asked about the PRESENT situation, so we don't include the 4 extra part-time workers discussed in the hypothetical situation.

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