Pipe A can fill a tank in 12 minutes; pipe B can fill it in 18 minutes, while pipe C can empty the full tank in 36 minutes. If all the pipes are opened simultaneously, how much time will it take to fill the empty tank completely?

Correct option is C

= $$\frac{1}{12}+\frac{1}{18}-\frac{1}{36} = \frac{3 +2-1}{36} =\frac{1}{9}$$ 

So, time to fill the tank = $$\frac{1}{\frac{1}{9}} = 9$$ minutes​