Two pipes $A$ and $B$ can fill a tank in $24$ $min.$ and $48$ $min.$ respectively. If both the pipes are opened simultaneously,after how much time (in $minutes$) $B$ should be closed so that the tank is full in $20$ $minutes$?

$A$ tank is filled by three pipes with uniform flow. The first two pipes operating simultaneously fill the tank in the same time during which the tank is filled by the third pipe alone. The second pipe fills the tank $5$ $hours$ faster than the first pipe and $4$ $hours$ slower than the third pipe. The time (in $hours$) required by the first pipe is:

$A$ big tanker can be filled by two pipes $A$ and $B$ in $60$ and $40$ $min$ respectively. What time will it take to fill an empty tanker if tap $B$ is used for half of the total time and both taps $A$ and $B$ together are used for the remaining half of the time?

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?

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.

Two pipes $A$ and $B$ can fill a tank in $20$ $minutes$ and $30$ $minutes$ respectively. If both the pipes are used together,then how long will it take to fill the tank? (in $minutes$)