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)

$A, B$ and $C$ are three pipes connected to a tank. $A$ and $B$ together fill the tank in $6$ $hrs$. $B$ and $C$ together fill the tank in $10$ $hrs$,and $C$ and $A$ together fill the tank in $7 \frac{1}{2}$ $hrs$. In how much time (in $hours$) will $A, B$ and $C$ fill the tank together?

$A$ reservoir is fitted with two pipes $A$ and $B$. Pipe $A$ can fill the reservoir $5$ $hours$ faster than pipe $B$. If both the pipes together fill the reservoir in $6$ $hours$,the reservoir will be filled by $A$ alone in:

$A$ tap drips at a rate of $1$ drop/sec. If $600$ drops make $100 \text{ ml}$,the number of litres wasted in $300$ days is:

Two taps can fill a tank in $20$ $min$ and $30$ $min$ respectively. When the tank was empty,both the taps were opened,and after some time,the first tap was closed. It took $18$ $min$ in total to fill the tank. After how much time (in $minutes$) from the beginning was the first tap closed?

Two pipes can fill a tank in $15$ $hours$ and $12$ $hours$ respectively,while a third pipe can empty it in $20$ $hours$. If the tank is empty and all three pipes are opened together,then in how much time will it take to fill the tank completely?