Working alone at its constant rate, machine A produces x boxes in 10 minutes and working alone at its constant rate, machine B produces 3x boxes in 5 minutes. How many minutes does it take machines A and B, working simultaneously at their respective constant rates, to produce 3x boxes?  Answers.
A) 3 minutes	B) 4 minutes	C) 4.2 minutes	D) 6 minutes	E) 12 minutes
Rate = Work / Time   Given Rate of Machine A = X / 10 min   Machine B Produces 3x boxes in 5 min hence , Machine B produces 4x boxes in 10 min .   Rate of Machine B = 6x / 10   we need tofind the combined time that machines A and B, working simultaneouslytakeat their respective constant rates   let's first find the combined Rate of Machine A and B   Rate of Machine A = X / 10 min + Rate of Machine B = 6x / 10 = 7X/10   Now combine Time = combine work needs to be done / 4.2 minutesombine Rate = 3x/7x * 10 = 4.2 Min   Ans: 4.2 minutes
<result>C</result>
Working alone at its constant rate, machine A produces x boxes in 10 minutes and working alone at its constant rate, machine B produces 3x boxes in 5 minutes. How many minutes does it take machines A and B, working simultaneously at their respective constant rates, to produce 3x boxes?