Difficulty level 700-800 (quant PS)

[Achints625]

Circular tyres X and Y start rotating simultaneously at uniform speeds. Tyres X makes 12 revolutions each minute and tyre Y makes 48 revolutions each minute. How long will it take from the time thay the tyres start rotating for the tyre Y to make 9 revolutions more than tyre X ?
A) 18 seconds
B) 15 seconds
C) 12 seconds
D) 10 seconds
E) 8 seconds

[RonPurewal]

per the forum rules,

* you need to cite the original source of the problem.

* you need to ask SPECIFIC QUESTION(s) about the problem[/b].
what do you understand?
what don't you understand?
where did you get stuck?
what approaches have you already tried?
etc.
you can't just post a problem and say "i don't know how to solve this."

thanks.

[VishnuA326]

It doesn't look like a 700 level problem. This should be 400 level problem.

Correct Answer is 15 Seconds as per below explanation.

X makes 12 revolutions in 1 Minute ...... (1)
Y makes 48 revolutions in 1 Minute ........ (2)

Ratio of revolution per minute between X and Y is 1:4. Now if you put answer keys in equation, In 15 seconds X will have 3 revolutions whereas Y will have 12 revolutions which is exactly 9 rounds more than x.

Hope my explanation is correct. @Experts : Please correct me if I am wrong.

Thanks,
Vishnu

[AndrewJ60]

Well you left out the equation it's a little difficult to understand for me personally. This is the way I did it:

KNOWN:
X = 12 per 60sec
Y = 48 per 60sec

Both can be simplified down to.

X= 1 per 5sec
Y= 4 per 5sec

Now step forward in time by 5 second intervals.

10 seconds: X = 2 and Y = 8 (not there yet)
15 seconds: X = 3 and Y = 12 ( That is a difference of 9 seconds, we have a winner)

15 seconds.

[tim]

I'm not going to delete the problem and discussion because it's pretty clear it's not a copyrighted GMAT problem. Still curious as to the source though, and whether the original poster has an actual question about the problem.

[RonPurewal]

yes. PLEASE do not just post a problem here and say "i have no idea how to do this problem."

if you really, genuinely have no idea how to do a problem, then, honestly, you won't derive any real value from an explanation of it -- even from a very, very good explanation.
worse yet, you'll be wasting the problem, since it will no longer be "fresh" for a time when you ARE capable of tackling it.

when you post a problem here, you should be able to articulate
• what approach(es) you've tried so far,
• what you ALREADY UNDERSTAND about the problem,
• what, SPECIFICALLY, you don't understand,
• where you're getting stuck.