If pipe $P$ can fill a tank in $p$ hours and pipe $Q$ can empty the same tank in $q$ hours (where $q > p$),and both pipes are opened simultaneously,in how many hours $(r)$ will the tank be filled?

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?

$A$ pipe can fill a tank with thrice the speed of another pipe. If both the pipes together take $36$ $min$ to fill an empty tank completely,then how much time will it take for the slower pipe to fill the tank alone?

One pipe can fill a tank three times as fast as another pipe. If together the two pipes can fill the tank in $36$ minutes,then the slower pipe alone will be able to fill the tank in (in minutes):

There are two taps to fill a tank while a third tap is used to empty it. When the third tap is closed,the first two taps can fill the tank in $10$ $minutes$ and $12$ $minutes$ respectively. If all three taps are opened,the tank is filled in $15$ $minutes$. If the first two taps are closed,in what time can the third tap empty the tank when it is full?

Three pipes $A, B$ and $C$ can fill a cistern in $6$ $hrs$. After working together for $2$ $hrs$,$C$ is closed and $A$ and $B$ fill the remaining part of the cistern in $8$ $hrs$. Find the time (in $hrs$) in which the cistern can be filled by pipe $C$ alone.