$A, B, C$ are pipes attached to a cistern. $A$ and $B$ can fill it in $20$ and $30$ minutes respectively,while $C$ can empty it in $15$ minutes. If $A, B, C$ are kept open successively for $1$ minute each,how soon will the cistern be filled? (in minutes)

An oil drum can be filled with oil in $40$ $min$ by a filling pipe. Another outlet pipe can empty the entire drum in $60$ $min$. The outlet pipe was opened when $\frac{2}{3}$ part of the drum was filled with oil and closed after $15$ $min$. What time will it take to fill the drum when the filling pipe is opened now?

Three taps $A, B$ and $C$ can fill a tank in $12, 15$ and $20$ hours respectively. If $A$ is open all the time and $B$ and $C$ are open for one hour each alternately,the tank will be full in? (in hours)

$A$ tap can fill the cistern in $8$ hours and another can empty it in $16$ hours. If both the taps are opened simultaneously,the time (in hours) to fill the tank is?

$A$ cistern has two taps which fill it in $12$ $minutes$ and $15$ $minutes$ respectively. There is also a waste pipe in the cistern. When all the pipes are opened,the empty cistern is filled in $20$ $minutes$. How long will the waste pipe take to empty a full cistern?

$A$ tank can be filled with water by two pipes $A$ and $B$ together in $36$ minutes. If the pipe $B$ was closed after $30$ minutes,the tank is filled in $40$ minutes. The pipe $B$ can alone fill the tank in?