‘A’ pipe can empty a tank in 20 minutes. The second pipe ‘B’ has a diameter twice as that of ‘A’. If both A & B pipe are attached to the tank how much time will be required to empty the tank ?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e. 
minutes
Suppose diameters of pipe 1st and pipe 2nd be 2unit and 4unit,so radius be 1unit and 2 unit respectively
so the ratio of area of pipe 1st and pipe 2nd will be π(1)2 : π (2)2 = 1: 4
here the ratio of efficiency of doing work of both the pipes is 1 : 4
let the efficiency of doing work of both the pipes is x and 4x
Efficiency × time = Total work
total work = x × 20 = 20x
as we know that the first pipe is doing negative work so when they both opened simultaneously total work will be done by the efficiency of 4x - x = 3x
20x/3x = minutes