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
Let r=1 and R=2.
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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