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Please correct my reasoning on DS question

Problem Solving — algebra and arithmetic (GMAT Focus Edition)
Expert replies
by shoot4greatness » Fri Jan 28, 2011 4:41 pm
Three machines, K, M, and P, working simultaneously and independently at their respective constant rates, can complete a certain task in 24 minutes. How long does it take Machine K, working alone at its constant rate, to complete the task?

(1) Machines M and P, working simultaneously and independently at their respective constant rates, can complete the task in 36 minutes.

(2) Machines K and P, working simultaneously and independently at their respective constant rates, can complete the task in 48 minutes.

My reasoning:

From the question, you can set up an equation 1/k + 1/m + 1/p = 1/24
(1) From this data, we can set up an equation 1/m + 1/p = 1/24. Plug it into the equation from the quesiton and solve for k. Sufficient.

(2) From this data, you can set up an equation 1/k + 1/p = 1/48 Plug it into the equation, we can only find the value of m.
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Source: — Quantitative Reasoning |

by shoot4greatness » Fri Jan 28, 2011 4:43 pm
Continueing

Since the question deals with 3 different distinct variable, we need 3 different distint equations. From the question and the data, we only have 2. Insufficient.

Answer A
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by Tani » Sat Jan 29, 2011 1:58 pm
You don't need three equations because you are not solving for each of the three variables. Clue a gives you a value for M and P, the remainder will be K. You can't separate M and P with this clue, but you can isolate K.

From the second clue you can pull out K and P, leaving M. You cannot isolate K or P.

Try thinking of M and P as a single machine that can complete the work in 36 minutes. You then have two machines (MP and K) and two equations.
Tani Wolff
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by VivianKerr » Fri Feb 04, 2011 11:46 am
Here's my thinking:

- Value question - only a definitive number for the amount of time it takes Machine K alone will be sufficient.

- Given info: together the 3 machines complete in 24 min.

(1) Machines M + P together do it in 36 min. We can solve for K. SUFF.

(2) Machines K + P together do it in 48. Here, we do not know what of the 48 min. we can attribute to K, and what we can attribute to P. NOT SUFF.
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