Two pipes $A$ and $B$ can fill a tank in $15$ hours and $20$ hours respectively,while a third pipe $C$ can empty the full tank in $25$ hours. All three pipes are opened in the beginning. After $10$ hours,pipe $C$ is closed. Find the total time (in hours) in which the tank will be full.

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?

Two pipes $A$ and $B$ can fill a cistern in $3$ $hours$ and $5$ $hours,$ respectively. Pipe $C$ can empty it in $2$ $hours.$ If all the three pipes are open,in how many $hours$ will the cistern be full?

$A$ tank can be filled by one tap in $20$ $min$ and by another tap in $60$ $min$. Both the taps are kept open for $5$ $min$ and then the $1$st tap is shut off. After this,how much more time (in $min$) will it take for the tank to be completely filled?

Two taps $A$ and $B$ can fill a tank in $10$ $hours$ and $12$ $hours$ respectively. If both taps are opened at $10 \text{ am}$,at what time (in $\text{pm}$) should tap $A$ be closed so that the tank is completely filled at exactly $4 \text{ pm}$?

Three pipes $A, B$ and $C$ can fill a tank from empty to full in $30$ minutes,$20$ minutes and $10$ minutes respectively. When the tank is empty,all the three pipes are opened. $A, B$ and $C$ discharge chemical solutions $P, Q$ and $R$ respectively. What is the proportion of solution $R$ in the liquid in the tank after $3$ minutes?