One pipe fills the water tower tank in 12 hours and the other in 20 hours.

One pipe fills the water tower tank in 12 hours and the other in 20 hours. How many hours will it take to fill the tank if both pipes are open?

We record the capacity of the water tower tank as 1.

Let’s find what part of the tank each pipe can fill in an hour of independent work.

To do this, we divide the capacity of the tank by the time of filling through each pipe.

We get:

1/12 = 1/12 of the tank per hour (1 pipe).

1/20 = 1/20 part of the tank per hour (2 pipe).

We find which part of the tank per hour both pipes will fill.

Let’s summarize the resulting parts.

1/12 + 1/20 = 5/60 + 3/60 = 8/60 = 2/15 part of the tank per hour.

This means that to fill the tank they will need:

1 / 2/15 = 15/2 = 7 1/2 hours.

Answer: 7.5 hours.