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Working alone at its constant rate, machine A can produce 240 pins in

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Math Revolution GMAT Instructor
Joined: 16 Aug 2015
Posts: 6369
GMAT 1: 760 Q51 V42
GPA: 3.82
Working alone at its constant rate, machine A can produce 240 pins in  [#permalink]

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10 Jan 2018, 01:02
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Difficulty:

35% (medium)

Question Stats:

69% (01:18) correct 31% (01:55) wrong based on 98 sessions

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[GMAT math practice question]

Working alone at its constant rate, machine A can produce 240 pins in x hours. Working alone at its constant rate, machine B can produce 240 pins in y hours. How many hours will it take machines A and B to produce 240 pins, when working together at their respective constant rates?

1) $$\frac{xy}{(x+y)}=4$$
2) $$x=20$$

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"Only $99 for 3 month Online Course" "Free Resources-30 day online access & Diagnostic Test" "Unlimited Access to over 120 free video lessons - try it yourself" examPAL Representative Joined: 07 Dec 2017 Posts: 707 Re: Working alone at its constant rate, machine A can produce 240 pins in [#permalink] Show Tags 10 Jan 2018, 03:02 MathRevolution wrote: [GMAT math practice question] Working alone at its constant rate, machine A can produce 240 pins in x hours. Working alone at its constant rate, machine B can produce 240 pins in y hours. How many hours will it take machines A and B to produce 240 pins, when working together at their respective constant rates? 1) $$\frac{xy}{(x+y)}=4$$ 2) $$x=20$$ Since (1) looks like a very technical expression, we'll look for a technical, equation-intensive solution. This is a Precise approach. Machine A produces 240/x pins per hour and machine B produces 240/y pins per hour. Together they produce 240/x + 240/y pins per hour which simplifies to 240(x+y)/xy pins per hour. Since rate * time = work then time = work/rate Therefore, to make 240 pins, they need to work for a total of 240 / (240(x+y)/xy) hours. This simplifies to xy/(x+y) hours. Therefore (1) is sufficient to answer the question and (2) is not. (A) is our answer. _________________ Sign up for 7-day free trial I am a CR Expert - Ask Me ANYTHING about CR I am a DS Expert - Ask Me ANYTHING about DS Watch free GMAT tutorials in Math, Verbal, IR, and AWA. GMAT test takers: Watch now the GMAC interview with the people who write the GMAT test! We discussed the chances of improving a GMAT score; how important the first questions on the test are; what to do if you don’t have enough time to complete a whole section; and more. You can watch all the action from the interview here. Math Revolution GMAT Instructor Joined: 16 Aug 2015 Posts: 6369 GMAT 1: 760 Q51 V42 GPA: 3.82 Re: Working alone at its constant rate, machine A can produce 240 pins in [#permalink] Show Tags 12 Jan 2018, 00:49 => Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution. The first step of the VA (Variable Approach) method is to modify the original condition and the question, and then recheck the question. We have $$\frac{1}{t} = \frac{1}{x} + \frac{1}{y} = \frac{(x+y)}{xy}$$ or $$t = \frac{xy}{(x+y)}$$, where t is the time that machines A and B take to produce 240 pins, when working together. The question asks for $$\frac{xy}{(x+y)}$$. Thus, only condition 1) is sufficient. Condition 2) is not sufficient as it only gives us information about $$x$$. Therefore, the answer is A. Answer: A _________________ MathRevolution: Finish GMAT Quant Section with 10 minutes to spare The one-and-only World’s First Variable Approach for DS and IVY Approach for PS with ease, speed and accuracy. "Only$99 for 3 month Online Course"
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Re: Working alone at its constant rate, machine A can produce 240 pins in &nbs [#permalink] 12 Jan 2018, 00:49
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