$A$ tap having diameter $d$ can empty a tank in $40$ $min$. How long will another tap having diameter $2d$ take to empty the same tank (in $min$)?

Two pipes $A$ and $B$ can fill a cistern in $4$ $minutes$ and $6$ $minutes,$ respectively. If these pipes are turned on alternately for $1$ $minute$ each,then how long will it take for the cistern to fill?

$A$ 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:

$A$ pipe can fill a tank in $x$ $h$ and another pipe can empty it in $y$ $(y > x)$ $h.$ If both the pipes are open,in how many hours will the tank be filled?

Three pipes $A, B$ and $C$ can fill a tank in $30 \ min$,$20 \ min$ and $10 \ min$,respectively. When the tank is empty,all the three pipes are opened. If $A, B$ and $C$ discharge chemical solutions $P, Q$ and $R$ respectively,then the part of solution $R$ in the liquid in the tank after $3 \ min$ is

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?