How do I answer this SAT Math question?

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

This question just requires a little bit of algebra. We can use the variable t for the time in number of hours each machine has been running. To find out how many hours it takes for the machines running together to make 900 bolts, we can set up this equation:

300t + 450t = 900
750t = 900
t = 1.2 hours

Now we have the time in hours, but we need it in MINUTES based on the question. We just have to convert 1.2 hours into minutes.

1.2 hours x 60 minutes per hour = 72 minutes

We can conclude that the answer is in fact B, 72 minutes!