**I guess the answer is B, but I can't seem to get there - can anyone explain?**

The smith metal company's old machine makes 300 bolts per hour. its new machine makes 450 bolts per hour. If both machines begin running at the same time, how many MINUTES will it take the two machines to make a total of 900 bolts?

a) 36

b) 72

c) 120

d) 144

e) 180