Pipes and Cistern

A can fill a tank in $12 \mathrm{hours}$ and B can fill the tank in $15 \mathrm{hours}$. Pipe A is opened for $4 \mathrm{hours}$. How much time will pipe B take to fill the remaining part of the tank?

Two taps A and B fill a tank in $15 \text{and} 20 \mathrm{hours}$ respectively, while a third tap C empties the filled tank in $25 \mathrm{hours}$. Initially, all three taps are opened and C has closed after $10 \mathrm{hours}$. In what time will the tank be filled?

A water tank is filled by tubes A and B in $5 \min$ and $6 \min$ respectively, while tap C can empty it in $8 \min$. If all the taps are opened simultaneously, then how much time will it take to fill the $\frac{3}{4}$ part of the trank?

The time is taken by pipe A and pipe B to fill the tank is $100 \mathrm{days}$, the time is taken by pipe B and pipe C to fill the same tank is twice the time taken by pipe A and pipe B together and the time taken by pipe A and pipe C is thrice the time taken by pipe B and pipe C to completely fill the tank. Find the time taken by pipe C to completely empty the tank.

The time is taken by pipe A to fill the tank is $20 \mathrm{hours}$, the time is taken by pipe B to fill the tank is double the time taken by tank A and the time is taken by pipe C is double the time taken pipe B to fill the tank. If pipe A is opened at $8:00 \mathrm{A}.\mathrm{M}.$, pipe B is opened at $9:00 \mathrm{A}.\mathrm{M}.$ and pipe C is opened at $10 \mathrm{A}.\mathrm{M}.$ Find the time at which the tank is completely filled.