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

rate of first = 1/20
rate of 2nd = 1/30
combined rate = 1/20 +1/30 = 1/12

time to fill at combined rate = 1/(1/12) = 12

It will take 12 minutes.