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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.