Pipe Q can fill the tank in 60 hours while pipe R may fill in 45 hours. Q and R pipes are opened together for 6 hours after which pipe W is also opened to empty the tank. All three pipes are opened simultaneously for 24 hours to reach the half-level mark. How much time (in hours) will pipe W alone take to empty the entire tank?

Correct option is C

Pipe Q efficiency = $$\frac{180}{60} = 3 \ unit/h$$  

Pipe R efficiency = $$\frac{180}{45} = 4 \ unit/h$$