ratio question

[kn2130]

Hi there,

I'm new to the Beat the GMAT community and I'm so psyched I found this place! Already, everyone has been so helpful. I need some help with this problem. I'm not sure if there's a typo in my book, or if i just am not getting this -- this is a fairly simple ratio question.

If cement, gravel, and sand are to be mixed in the ratio of 3:5:7 respectively, and 5 tons of cement are available, how many tons of the mixture can be made?

a) 15
b) 20
c) 25
d) 30
e) 75

answer: C

Here's the book's explanation
Since the ratio of cement to gravel to sand is 3:5:7, for every 3 portions of cement we put in, we get 3+5+7 or 15 portions of mixture. Therefore the recipes gives us 15/3 or 5 times as much mixture as cement. We have 5 tons of cement available, so we can make 5x5 or 25 tons of the mixture.

This is in the Kaplan Math Workbook.

[sars72]

The explanation seems fairly straightforward, maybe your mind is blocked and you are too tired to see what they're trying to explain - this has happened to me on numerous occasions.

I'll show another, slightly more tedious way of solving the problem. Hopefully it'll help:

ratio -> 3:5:7

we have 5 of cement, to get this in the ratio, we have to multiply 3 by (5/3) since 3*5/3 = 5

so now that we have 5 for cement, we have to multiply the other units in the ratio with the same value (5/3)

-> gravel = 5*(5/3) = 25/3
and sand = 7*(5/3) = 35/3

Adding all the 3, we have 5 + 25/3 + 35/3 = 5 + 60/3 = 5 + 20 = 25

Please let me know if you're still having problems with this.

[kn2130]

oh my goodness... ha! yes, i see it. i was just blocked. I was "looking" for something else. (Plus, there was a typo earlier in the book that had me working on a problem for way too long, and now I'm paranoid there are going to be more.) Thanks so much!