Consider the tank to be 180 units.

Hence rate at which Pipe A fills the tank is 60 units(since it fills the tank in 3 hours)

Also, rate at which Pipe B empties the tank is 90 units(since it empties the tank in 2 hours)

It is given that the tank is 2/3rd's full or has 120 units of water in it.

We need to find the time to empty it completely given both the pipes are on.

Both the pipes being on, will reduce 30 units(+60 - 90) an hour

120 - 30(x) = 0

or x=4(Option C)

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