Q30:

Machine A, operating alone at its constant rate, produces 500 feet of a particular fiber in 2 hours.  Machine B, operating alone at its constant rate, produces 500 feet of the same fiber in 3 hours.  Machine C, operating alone at its constant rate, produces 500 feet of the same fiber in 5 hours.  How many hours will it take machines A, B, and C, operating together at their respective constant rates, to produce 1,000 feet of the fiber?

key: A 60/31

我怎么算都是20呀，可答案里都是没有，怎么回事？这题很简单的呀

把500feet 看成单位1,1000feet就是单位2.

所以, ANSWER = 2/(1/2+1/3+1/5) = 60/31

我猜LZ是小地方做错了吧,常有的事.