GMAT Problem Solving (PS) | Re: Machine A, operating alone at its constant rate, produces 500 feet of

Lets assume the 500 ft fibre work to be = 6 units (6 is LCM of 2, 3, 6)

Then, Machine A's per hour work = 3 units
Machine B's per hour work = 2 units
Machine C's per hour work = 1 unit

(A+B+C) combined per hour work = 3+2+1 = 6 units
Thus together they can do 500 ft fibre work in 1 hour, so to do double the work (1000 ft), they would need double time, i.e., 2 hours. Hence C answer

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