M7MBA wrote:A company has two models of computers, model M and model N. Operating at a constant rate, a model M computer can complete a certain task in 18 minutes and a model N computer can complete the same task in 9 minutes. If the company used the same number of each model of computer to complete the task in 1 minute, how many model M computers were used?
(A) 2
(B) 3
(C) 6
(D) 9
(E) 12
Let the task = 18 units.
Since M takes 18 minutes to complete the 18-unit task, M's rate = w/t = 18/18 = 1 unit per minute.
Since N takes 9 minutes to complete the 18-unit task, N's rate = w/t = 18/9 = 2 units per minute.
Combined rate for M+N = 1+2 = 3 units per minute.
For the 18-unit task to be completed in 1 minute, the required rate = w/t = 18/1 = 18 units per minute.
Since the rate for M+N = 3 units per minute, the number of M+N's required to produce 18 units per minute = (required rate)/(rate for M+N) = 18/3 = 6.
The correct answer is
C.
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