BTGmoderatorLU wrote:Source: e-GMAT
A and B can complete a task in 20 days. B and C can complete the same task in 30 days. If A and C can complete the task in 40 days, arrange the efficiencies of A, B, and C in descending order.
A. A>B>C
B. A>C>B
C. B>A>C
D. B>C>A
E. C>A>B
Alternate approach:
Let the job = 240 units.
Since A+B take 20 days to complete the 240-unit job, A+B = 240/20 = 12 units per day.
Since B+C take 30 days to complete the 240-unit job, B+C = 240/30 = 8 units per day.
Since A+C take 40 days to complete the 240-unit job, A+C = 240/40 = 6 units per day.
Thus:
(A+B) - (B+C) = 12-8.
A-C = 4 units per day.
(A-C) + (A+C) = 4+6
2A = 10
A = 5 units per day.
Since A-C = 4 units per day, and A = 5 units per day, C = 1 unit per day.
Since B+C = 8 units per day, and C = 1 unit per day, B = 7 units per day.
Since B=7, A=5 and C=1, B>A>C.
The correct answer is
C.
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