Machine A and machine B process the same work at different rates. Machine C processes work as fast as Machines A and B combined. Machine D processes work three times as fast as Machine C; Machine D’s work rate is also exactly four times Machine B’s rate. Assume all four machines work at fixed unchanging rates. If Machine A works alone on a job, it takes 6 hours and 20 minutes. If all four machines work together on the same job simultaneously, how many minutes will it take all of them to complete it? Select:	A) 15	B) 17	C) 19	D) 21	E) 24
C = A + B   D = 3C = 3(A+B) = 4B   Then B = 3A and C = 4A   The combined rate of the four machines is A + 3A + 4A + 12A = 20A   Machine A can complete the work in 380 minutes, so its rate is 1/380 of the work per minute.   The combined rate is 20/380 = 1/19 so the work will be completed in 19 minutes.   The answer is 19.
<result>C</result>
Machine A and machine B process the same work at different rates. Machine C processes work as fast as Machines A and B combined. Machine D processes work three times as fast as Machine C; Machine D’s work rate is also exactly four times Machine B’s rate. Assume all four machines work at fixed unchanging rates. If Machine A works alone on a job, it takes 6 hours and 20 minutes. If all four machines work together on the same job simultaneously, how many minutes will it take all of them to complete it?