Bunuel wrote:

A metal company's old machine makes bolts at a constant rate of 100 bolts per hour.The company's new machine makes bolts at a constant rate of 150 bolts per hour. If both machines start at the same time and continue making bolts simultaneously, how many minutes will it take the two machines to make a total of 300 bolts?

A. 36

B. 72

C. 120

D. 144

E. 180

First way: In an hour you will make: 100+150 = 250.

So in two -> 500 > 300.

Which means C,D,E falls immediately.

Also, everything under an hour, A falls.

-> and the answer is B. 72.

Second way:

12 minutes = 1/5 of an hour, so that's 150/5 + 100/5 = 30+20 = 50.

300 would need 6 times that.

So 12*5 = 72 minutes.