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by success1111 » Thu Apr 30, 2009 7:53 pm
Machine A, operating alone at its constant rate, produces 500 feet of a particular fiber in 2 hours. Machine B, operating alone at its constant rate ,produces 500 feet of the same fiber in 3 hours. Machine C, operating alone at its constant rate. Produces 500 feet of the same fiber in 5 hours. How many hours will it take machines A. B, and C, operating together at their respective constant rates, to produce 1000 feet of the fiber?

A. 60/31
B. 15/7
C. 25/11
D. 10/2
E. 20/3
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by dumb.doofus » Thu Apr 30, 2009 9:11 pm
A i.e. 60/31

Fiber produced by Machine A in one hour = 250
Fiber produced by Machine B in one hour = 500/3
Fiber produced by Machine C in one hour = 100

Total fiber produced in an hour by all machines = 1550/3

so 1000 ft can be produced in 3*1000/1550 = 60/31 hours.
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by mixpanda » Thu Apr 30, 2009 10:20 pm
I think the answer is A as well.

If we were seeing how long it would take them to produce 500 feet together it would be:

1/2 + 1/3 + 1/5 = 1/T

But since they are making 1000 feet:

1/4 + 1/6 + 1/10 = 1/T
15/160 + 10/60 + 6/60 = 1/T
31/60 = 1/T

Cross-multiply => T=60/31
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by VP_Jim » Fri May 01, 2009 7:57 am
Or, you can recognize that combined they do a little over 500 feet per hour. Thus, to make 1,000 feet, it will take a little under two hours. That saves you a little math, and still draws you to the correct answer since A is the only one under two hours.

Nonetheless, it's good to know how to do the math, too.
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by success1111 » Fri May 01, 2009 7:43 pm
VP_Jim wrote:Or, you can recognize that combined they do a little over 500 feet per hour. Thus, to make 1,000 feet, it will take a little under two hours. That saves you a little math, and still draws you to the correct answer since A is the only one under two hours.

Nonetheless, it's good to know how to do the math, too.
Yeah.Thanks to dumb.doofus and mixpanda

VP_JIM,Thanks. i know how to solve most of these questions. My strategy for quantitative section of GMAT is that any practice question i cant solve in one minutes, forty seconds, i will post it here for experts like you to help me out.I strongly believe that almost all the quantitative sections in GMAT can be solved in less than 2 minutes. i am practising less than 2 minutes to accommodate any circumstances during the real test.
Trust but verify.
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