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?	Choose the correct choice from the following options.
A) 3 minutes	B) 4 minutes	C) 4.2 minutes	D) 6 minutes	E) 12 minutes

Rationale: "rate = work / time given rate of machine a = x / 10 min machine b produces 3 x boxes in 5 min hence , machine b produces 4 x boxes in 10 min . rate of machine b = 6 x / 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 = 6 x / 10 = 7 x / 10 now combine time = combine work needs to be done / combine rate = 3 x / 7 x * 10 = 4.2 min ans : c"
Answer: C