A pipe can fill a tank in 9 hours. Another pipe can empty the filled tank in 36 hours. If both the pipes are opened simultaneously, then the time (in hours) in which the tank will be two-third filled, is:

Correct option is D

Given:

The first pipe can fill the tank in 9 hours.

The second pipe can empty the tank in 36 hours.  

Solution:

Capacity of Tank = LCM of 9 and 36 = 36 unit

Efficiency of A = $$\frac{36}{9} = 4$$ unit\day

Efficiency of other pipe  = $$\frac{36}{36} = -1$$ 

Combine efficiency = 4 - 1 = 3 

They will fill the two- third  tank = $$\frac{2}{3}\times \frac{36}{3 } = 8$$ hours ​