If V = 12R / (r + R) , then R =
A. Vr / (12 - V)
B. Vr + V /12
C. Vr - 12
D. V / r - 12
E. V (r + 1) /12
I got V (r+R)= 12R
Vr + VR=12R
Vr =12R - VR
Vr = R(12-V)
R(12 -V)/ Vr
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Value of R
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chacha0212
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gmatcracker0123
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Let r=1 and R=2.chacha0212 wrote:If V = 12R / (r + R) , then R =
A. Vr / (12 - V)
B. Vr + V /12
C. Vr - 12
D. V / r - 12
E. V (r + 1) /12
Then V = (12*2)/(1+2) = 8.
The question stem asks for the value of R=2. This is our target.
Now plug r=1 and V=8 into the answer choices to see which yields our target of R=2.
A. Vr / (12 - V) = (8*1)/(12-8) = 8/4 = 2.
B. Vr + V /12 = (12*1 + 8)/12 = 20/12 = 5/3.
C. Vr - 12 = 8*1 - 12 = -4.
D. V / r - 12 = 8/(1-12) = 8/-11.
E. V (r + 1) /12 = 8(1+1) / 12 = 16/12 = 4/3.
The correct answer is A.
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Brent@GMATPrepNow
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Take V = 12R / (r + R)chacha0212 wrote:If V = 12R / (r + R) , then R =
A. Vr / (12 - V)
B. Vr + V /12
C. Vr - 12
D. V / r - 12
E. V (r + 1) /12
Multiply both sides by (r + R) to get: V(r + R)= 12R
Expand: Vr + VR = 12R
Subtract VR from both sides: Vr = 12R - VR
Factor R from right side: Vr = R(12 - V)
Divide both sides by (12 - V) to get: Vr/(12 -V) = R
Answer: A
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Brent@GMATPrepNow
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Everything is perfect up until you got to Vr = R(12-V)chacha0212 wrote:If V = 12R / (r + R) , then R =
A. Vr / (12 - V)
B. Vr + V /12
C. Vr - 12
D. V / r - 12
E. V (r + 1) /12
I got V (r+R)= 12R
Vr + VR=12R
Vr =12R - VR
Vr = R(12-V)
R(12 -V)/ Vr
At this point, we need to DIVIDE both sides by (12 - V) to get: Vr/(12 - V) = R
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Matt@VeritasPrep
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One caveat here: you have to know that V ≠12; the GMAT would specify that in the question stem.
