Question

one pipe can fill a tank in 20 minutes, while another takes 30 minutes to fill the same tank. How long would it take the two pipes together to fill the tank?

Answer 1

look at rates per minute
pipe 1: rate of 1/20 per minute
pipe 2: rate of 1/30 per minute

both pipes: 1/20 + 1/30 per minute
--> 1/20 + 1/30 = 3/60 + 2/60 = 5/60 = 1/12

--> 12 minutes