$A$ pipe $P$ can fill a tank in $12$ $min$ and another pipe $R$ can fill it in $15$ $min.$ But,the $3$rd pipe $M$ can empty it in $6$ $min.$ The $1$st two pipes $P$ and $R$ are kept open for $5$ $min$ in the beginning and then the $3$rd pipe is also opened. In what time (in minutes) is the tank emptied?

Three taps $P, Q$ and $R$ separately can fill a tank completely in $4, 8$ and $12$ hours respectively. Another tap $S$ can empty the filled tank in $10$ hours. Which of the following combinations will fill the empty tank in the least amount of time?

Two pipes can fill a tank in $20$ and $24$ $minutes$ respectively and a waste pipe can empty $3$ $gallons$ per $minute$. All the three pipes working together can fill the tank in $15$ $minutes$. The capacity of the tank is?

$A$ swimming pool has $3$ drain pipes. The first two pipes $A$ and $B$,operating simultaneously,can empty the pool in half the time that $C$ (the $3rd$ pipe) alone takes to empty it. Pipe $A$,working alone,takes half the time taken by pipe $B$. Together they take $6$ $hours$ $40$ $minutes$ to empty the pool. Time taken by pipe $A$ to empty the pool,in hours,is

One tap can fill a cistern in $2$ hours and another can empty the cistern in $3$ hours. How long will it take to fill the cistern if both the taps are opened (in $\text{hours}$)?

Three pipes $A, B$ and $C$ can fill a tank in $6$ $hours$. After running them together for $2$ $hours$, the tap $C$ is closed and $A$ and $B$ fill the remaining part in $7$ $hours$. How much time (in $hours$) would $C$ take to fill the tank alone?