sownthar wrote:Machine M takes 5 hours to complete a certain order, and Machine N takes 2 hours to complete half of that same order. How many hours would it take both machines, working simultaneously, to complete a double order?
A. 10/7
B. 5/3
C. 20/9
D. 20/7
E. 40/9
OA is E
Please explain your approach to this problem. Expecting to know different ways to solve.
Let the order = 20 units.
M's rate = 20/5 = 4 units per hour.
Since the time for N to complete half the order (10 units) is 2 hours, N's rate = 10/2 = 5 units per hour.
Double the order = 40 units.
When elements work together, we add their rates.
Thus, M+N's combined rate = 4+5 = 9 units per hour.
Thus, the time for M+N to complete a double order = 40/9.
The correct answer is
E.
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