Working alone at its constant rate, machine A produces x boxes in 5 minutes and working alone at its constant rate, machine B produces 2x boxes in 10 minutes. How many minutes does it take machines A and B, working simultaneously at their respective constant rates, to produce 3x boxes?  Choose the most appropriate option: A) 3 minutes B) 7.5 minutes C) 5 minutes D) 6 minutes E) 12 minutes

Rationale: "rate = work / time given rate of machine a = 2 x / 10 min machine b produces 2 x boxes in 10 min hence , machine b produces 2 x boxes in 10 min . rate of machine b = 2 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 = 2 x / 10 min + rate of machine b = 2 x / 10 = 4 x / 10 now combine time = combine work needs to be done / combine rate = 3 x / 4 x * 10 = 7.5 min ans : b"
Answer: B