Two pipes $A$ and $B$ can fill a cistern in $20$ $minutes$ and $25$ $minutes$ respectively. Both are opened together,but after $5$ $minutes,$ $B$ is turned off. How much time (in $minutes$) longer will the cistern take to fill?

Two taps $P$ and $Q$ can fill a water tank in $12$ $min$ and $15$ $min$ respectively. They are opened together,but after $3$ $min$,tap $P$ is closed. How much more time will $Q$ take to fill the remaining part of the tank?

An empty tank can be filled by pipe $A$ in $4$ $h$ and by pipe $B$ in $6$ $h$. If the two pipes are opened for $1$ $h$ each alternately,starting with pipe $A$,then in how much time will the tank be filled?

$A$ tank has two pipes. The first pipe can fill it in $45$ minutes and the second can empty it in $1$ hour. In what time will the empty tank be filled if the pipes are opened one at a time in alternate minutes?

$18$ pumps can fill a reservoir of capacity $1440 \text{ kilo liter}$ in $5 \text{ hours}$. How many $\text{hours}$ are required to fill a reservoir of capacity $1920 \text{ kilo liter}$ by $8$ pumps?

Two inlet pipes can fill a cistern in $10$ $hours$ and $12$ $hours$ respectively,and an outlet pipe can empty $80$ $gallons$ of water per hour. All three pipes working together can fill the empty cistern in $20$ $hours$. What is the capacity (in $gallons$) of the tank?