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if rs is not equal to 0, is 1/r + 1/s = 4? 1) r+s = 4rs 2) r [#permalink]
 11 Jun 2007, 22:19
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if rs is not equal to 0, is 1/r + 1/s = 4?
1) r+s = 4rs
2) r = s
Please explain your solution
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GMAT Club Legend
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St1:
r = 4rs-s = s(4r-1)
Insufficient.
St2:
1/r + 1/s = 2/r or 2/s. Insufficient.
Using both, we have, 2s = 4s^2
2s(1-2s) = 0
s = o (invalid) or s = 1/2 = r. So we can answer the question now. Sufficient.
Ans C
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ggarr wrote: if rs is not equal to 0, is 1/r + 1/s = 4?
1) r+s = 4rs
2) r = s
Please explain your solution
The answer is A.
1) (r+s) / rs = 4
=> (r/rs) + (s/rs) = 4
=> (1/r) + (1/s) = 4
SUFFICIENT
2) plug in r=s=1/2, you get 4=4. But plug in r=s=1, you get 2=4. INSUFFICIENT.
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bkk145 wrote: ggarr wrote: if rs is not equal to 0, is 1/r + 1/s = 4?
1) r+s = 4rs
2) r = s
Please explain your solution The answer is A.
1) (r+s) / rs = 4
=> (r/rs) + (s/rs) = 4
=> (1/r) + (1/s) = 4
SUFFICIENT2) plug in r=s=1/2, you get 4=4. But plug in r=s=1, you get 2=4. INSUFFICIENT. gar!! failed to see that... 
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Manager
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For me the answer is A
r+s=4rs
dividing both sides by rs
r/rs+s/rs=4
1/r+1/s=4 (Sufficient)
r=s
Different values of r or s suggest that 1/r+1/s may or may not be =4 thus insufficient
so Answer is A
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Director
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ywilfred wrote: bkk145 wrote: ggarr wrote: if rs is not equal to 0, is 1/r + 1/s = 4?
1) r+s = 4rs
2) r = s
Please explain your solution The answer is A.
1) (r+s) / rs = 4
=> (r/rs) + (s/rs) = 4
=> (1/r) + (1/s) = 4
SUFFICIENT2) plug in r=s=1/2, you get 4=4. But plug in r=s=1, you get 2=4. INSUFFICIENT. gar!! failed to see that... why can't we just answer this question w/the info provided in the stem? did we really need additional info to figure that out? 1/r + 1/s = 4 => s/rs + r/rs = 4 => 1/r + 1/s = 4
does this make sense?
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Manager
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Well we could certainly have done it that way Fig if the stem had told us that 1/r+1/s is indeed equal to 4. The stem is asking whether the expression is a true equation or not. And to validate the stem we are provided with two different sets of information which are statements A & B.
Actually this is how i had made D the correct option by putting r=s in the stem. But as u see, stem is not stated to be true, it is asked wherther it is true or not.
I hope this explains ur question.
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Director
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it does. thanks. I made this more difficult than I should have. why are you calling me Fig?
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Manager
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sorry dude
u see when the boss is just around the corner, then such mistakes are bound to happen. Especially when ur boss is more like a blood hound, who only wnats ur blood instead of ur sweat. 
And yes u did make it more difficult. It happens to me too. simple solution is in fornt of me, but i start making complex equations.
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ggarr wrote: it does. thanks. I made this more difficult than I should have. why are you calling me Fig?
Because I'm the one who will do such thing 
By the way, I'm also with (A)  ...... because neither r nor s could be equal to 0..... We can thus divide the equation by rs.
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shahrukh wrote: :lol:
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ggarr wrote: if rs is not equal to 0, is 1/r + 1/s = 4?
1) r+s = 4rs
2) r = s
Please explain your solution
From the stem we have that (r+s)/rs = 4
1) (r+s)/rs= 4rs/rs = 4, SUF
2) r=s, from the stem => 1/r, but since we do not know how much does r worth we also do not know how much 1/r worths, INSUF.
A
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