Pipe A B and C are kept open and together fill a tank in t minutes Pipe A is kept open throughout pi...........

Explanation:

A is kept open for all t minutes and fills one-third the tank. Or, A should be able to fill the entire tank in '3t' minutes.
A and B together can fill the tank completely in t minutes.
A alone can fill it in 3t minutes.
A and B together can fill 1/t of the tank in a minute.
A alone can fill 1/3t of the tank in a minute.
So, in a minute, B can fill 1/t - 1/3t = 2/2t
Or, B takes 3t/2 minutes to fill an entire tank.
To fill one-third the tank, B will take t/2 minutes. B is kept open for 10 minutes.
t/2 = t - 10, t = 20 minutes.
A takes 60 minutes to fill the entire tank, B takes 30 minutes to fill the entire tank.
A is kept open for all 20 minutes. B is kept open for 10 minutes.
C, which is kept open for 8 minutes also fills one-third the tank.
Or, C alone can fill the tank in 24 minutes.