A tank is fitted with 8 pipes, some that fill the tank and others that empty the tank. Each of the fill pipes fills the tank in 8 hours, while each of the waste pipes empties it in 6 hours. lf all the pipes are kept open when the tank is full, the tank will be completely drained in 6 hours. How many of these 8 pipes are fill pipes?

Correct Answer: 4 fill pipes

Let the number of fill pipes be ‘n’. Therefore, there will be (8 – n) waste pipes.

Each fill pipe fills 1/8th of the tank in an hour. So, “n” fill pipes will fill n/8th of the tank in an hour.

Each waste pipe drains 1/6th of the tank in a hour.

So, 8-n/6 waste pipes will drain 8−n6th of the tank in an hour.

Keeping these 8 pipes open results in 1/6th of the tank being drained every hour.

Hence, n/8−8−n/6=−1/6

Solving for n, we get n = 4.

Therefore, there are 4 fill pipes and 4 waste pipes.