$A$ cistern has two pipes. One can fill it in $8 \, h$ and the other can empty it in $5 \, h$. In how many $hours$ will the cistern be emptied if both the pipes are opened together when $\frac{3}{4}$ of the cistern is already full of water?

$A$ water tank has two pipes. The empty tank is filled in $12$ $min$ by the $1$st pipe and the full tank is emptied by the $2$nd pipe in $20$ $min$. What is the time required to fill the $1/2$ full tank when both pipes are in action (in $min$)?

If a pipe fills a tank in $6 \ h$,then what part of the tank will the pipe fill in $1 \ h$?

Two pipes $A$ and $B$ can fill a tank in $15$ minutes and $20$ minutes respectively. Both the pipes are opened together but after $4$ minutes,pipe $A$ is turned off. What is the total time required to fill the tank?

Two pipes $A$ and $B$ can fill a tank in $24$ $minutes$ and $32$ $minutes$ respectively. If both the pipes are opened simultaneously,after how much time (in $minutes$) should $B$ be closed so that the tank is full in $18$ $minutes$?

Two pipes $A$ and $B$ can fill a cistern in $18$ $minutes$ and $9$ $minutes$ respectively,while a third pipe $C$ can empty the full tank in $5$ $minutes$. $A$ and $B$ are kept open for $4$ $minutes$ in the beginning and then $C$ is also opened. In what time (in $minutes$) is the cistern emptied?