$A$ tank has a leak which would empty it in $8$ $hrs$. $A$ tap is turned on which admits $6$ $litres$ a minute into the tank,and it is now emptied in $12$ $hrs$. How many litres does the tank hold?

Three taps $A, B$ and $C$ can fill a tank in $12, 15$ and $20$ hours respectively. If $A$ is open all the time and $B$ and $C$ are open for one hour each alternately,the tank will be full in? (in hours)

$A$ tap can fill a cistern in $8$ $hours$ and another can empty it in $16$ $hours$. If both the taps are opened simultaneously,the time (in $hours$) to fill the tank is?

Pipes $P$ and $Q$ can fill a tank in $10$ and $12$ $h$, respectively, and pipe $C$ can empty it in $6$ $h$. If all the three are opened at $7$ $am$, at what time will one-fourth of the tank be filled?

Two pipes $A$ and $B$ can fill a tank in $24$ $min.$ and $48$ $min.$ respectively. If both the pipes are opened simultaneously,after how much time (in $minutes$) $B$ should be closed so that the tank is full in $20$ $minutes$?

$A, B$ and $C$ are three pipes connected to a tank. $A$ and $B$ together fill the tank in $6 \text{ h}$,$B$ and $C$ together fill the tank in $10 \text{ h}$ and $A$ and $C$ together fill the tank in $12 \text{ h}$. In how much time will $A, B$ and $C$ fill up the tank together?