Bunuel wrote:
Machine A produces nails at a uniform rate of 120 every 10 seconds, and Machine B produces nails at a uniform rate of 100 every 15 seconds. If the two machines run simultaneously, how many seconds will it take for them to produce a total of 140 nails?
A. 7.0
B. 7.5
C. 8.0
D. 8.5
E. 9.0
Efficiency of machine A is 120/10 = 12
Efficiency of machine B is 100/15 = 20/3
Combined efficiency of Machine A and B is 36 + 20/3 =56/3
Time required to produce 140 nails is 140*3/56 = 10*3/4 = 7.5, Answer must
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