$2$ pipes $A$ and $B$ can fill a tank separately in $60$ $min$ and $70$ $min$ respectively. There is a third pipe attached to the bottom of the tank to empty it. The tank is filled in $60$ $min$ when all three pipes are opened. In how much time $(min)$ will the third pipe alone take to empty the tank?

$A$ boy and a girl together fill a cistern with water. The boy pours $4$ litres of water every $3$ minutes and the girl pours $3$ litres every $4$ minutes. How much time (in minutes) will it take to fill $100$ litres of water in the cistern?

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?

$A$ 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 pipes $A, B$ and $C$ can fill a tank in $6$ $hours$. After working together for $2$ $hours$,$C$ is closed and $A$ and $B$ fill the remaining part in $7$ $hours$. The number of $hours$ taken by $C$ alone to fill the tank is?

Pipe $A$ can fill a tank in $4$ $hours$ and pipe $B$ can fill it in $6$ $hours$. If they are opened on alternate hours and if pipe $A$ is opened first,then in how many hours will the tank be full?