Pipe A can fill a tank in 10 hrs and pipe B can fill it in 8 hrs. If both the pipes are opened in the empty tank.There is an outlet pipe in 3/4 th of the tank. In how many hours will it be fill 3/4th of that tank?
Select.
A) 3 hr B) 3.10 hr C) 3.15 hr D) 3.20 hr E) 3.30 hr

Rationale: part filled a in 1 hr = ( 1 / 10 ) part filled b in 1 hr = ( 1 / 8 ) part filled by ( a + b ) together in 1 hr = ( 1 / 10 ) + ( 1 / 8 ) = 18 / 80 so , the tank will be full in 80 / 18 hrs . time taken to fill exact 3 / 4 th of the tank = ( 80 / 18 ) * ( 3 / 4 ) = 3.20 hrs answer : d
Answer: D