Question

Pipe a can fill a tank in 12 minutes and pipe b can fill it in 15 minutes, pipe c can empty the tank in 6 minutes. if pipe a and b are opened for 5 minutes. and then c is also opened, then in how much time will the tank get empty?

Answer 1

Pipe A in 1 minute can fill 1/12 of the tank
Pipe B in 1 minute can fill 1/15 of the tank
Pipe C in 1 minute can Empty 1/6 of the tank

Pipe A & B in 5 minutes fill: 5*(1/12+1/15) = 5*9/60 = 3/4 of the tank

Pipe A,B& C in 1 minute: 1/12 + 1/15 - 1/6 = -1/60 or empty 1/60 of the tank.

Till 5 minutes, 3/4 was full, from 6th minute 1/60 is emptied
1/60 emptied=== 1 minute
3/4 emptied == x minutes
x =3*60/4 = 45 minutes.
So in another 45 minutes (after 5 minutes of A &B 's work),the tank will be emptied.
Hope this helps