$A$ cistern has two taps which fill it in $12$ $minutes$ and $15$ $minutes$ respectively. There is also a waste pipe in the cistern. When all the pipes are opened,the empty cistern is filled in $20$ $minutes$. How long will the waste pipe take to empty a full cistern?

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?

Pipe $A$ can fill a tank in $4$ $hours$ and pipe $B$ can fill it in $6$ $hours$. If they are opened on alternate hours and if pipe $A$ is opened first,then in how many hours will the tank be full?

Two taps can separately fill a cistern in $10$ $minutes$ and $15$ $minutes$ respectively. When the waste pipe is open,they can together fill it in $18$ $minutes$. The waste pipe can empty the full cistern in:

$A$ water tank has three taps $A$,$B$ and $C$. Tap $A$,when opened,can fill the water tank alone in $4$ $hours$. Tap $B$,when opened,can fill the water tank alone in $6$ $hours$. Tap $C$,when opened,can empty the water tank alone in $3$ $hours$. If taps $A$,$B$ and $C$ are opened simultaneously,how long will it take to fill the tank completely? (in $hours$)

If pipe $P$ can fill a tank in $p$ hours and pipe $Q$ can empty the same tank in $q$ hours (where $q > p$),and both pipes are opened simultaneously,in how many hours $(r)$ will the tank be filled?