Two pipes can fill a tank in $15$ $hours$ and $12$ $hours$ respectively,while a third pipe can empty it in $20$ $hours$. If the tank is empty and all three pipes are opened together,then in how much time will it take to fill the tank completely?

$A$ pipe can fill a tank with water in $3 \, h$. Due to a leak at the bottom,it takes $3 \frac{1}{2} \, h$ to fill it. In what time (in $hours$) will the leak empty the fully filled tank?

$A$ tap can fill a cistern in $40$ $min$ and a second tap can empty the filled cistern in $60$ $min$. By mistake, without closing the second tap, the first tap was opened. In how many $minutes$ will the empty cistern be filled?

$A$ cistern has three pipes $A, B$ and $C$. Pipes $A$ and $B$ can fill it in $3 \, h$ and $4 \, h$, respectively, while pipe $C$ can empty the completely filled cistern in $1 \, h$. If pipes are opened in order at $3:00 \, pm$, $4:00 \, pm$ and $5:00 \, pm$, respectively, at what time will the cistern be empty?

$A$ tank can be filled by pipe $A$ in $2$ $h$ and pipe $B$ in $6$ $h$. At $10$ $am$ pipe $A$ was opened. At what time will the tank be filled if pipe $B$ is opened at $11$ $am$?

$A$ large tanker can be filled by two pipes $A$ and $B$ in $60$ $minutes$ and $40$ $minutes$ respectively. How many minutes will it take to fill the tanker from an empty state if pipe $B$ is used alone for half the time and pipes $A$ and $B$ are used together for the other half?