$A$ 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 $11$ $am$?

Three taps $A, B$ and $C$ together can fill an empty cistern in $10$ $min.$ The tap $A$ alone can fill it in $30$ $min$ and the tap $B$ alone can fill it in $40$ $min.$ How long will the tap $C$ alone take to fill it? (in $minutes$)

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?

$A$ 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?

One tap can fill a cistern in $2$ $hours$ and another can empty the cistern in $3$ $hours.$ How long will they take to fill the cistern if both the taps are opened?

$A$ tap can fill an empty tank in $12$ $hours$ and another tap can empty the half-filled tank in $10$ $hours$. If both the taps are opened together,then how much time (in $hours$) will it take to fill an empty tank half-filled?