$18 \text{ litre}$ of pure water was added to a vessel containing $80 \text{ litre}$ of pure milk. $49 \text{ litre}$ of the resultant mixture was then sold,and some more quantity of pure milk and pure water was added to the vessel in the respective ratio of $2:1$. If the resultant respective ratio of milk and water in the vessel was $4:1$,what was the quantity of pure milk added in the vessel? (in $\text{litre}$)

$A$ $12 \text{ litres}$ cylinder contains a mixture of oxygen and nitrogen,the volume of oxygen being $40 \%$ of the total volume. $A$ few litres of the mixture is released and an equal amount of nitrogen is added. Then,the same amount of the mixture as before is released and replaced by nitrogen for the $second$ time. As a result,the oxygen content becomes $10 \%$ of the total volume. How many $litres$ of the mixture is released each time?

$A$ container contained $80$ $litre$ of milk. From this container $8$ $litre$ of milk was taken out and replaced by water. This process was further repeated $3$ times. How much milk is now contained in the container? (in $litre$)

$4 \, kg$ of potato at $Rs. \,5$ per $kg$ is mixed with $8 \,kg$ of potato at $Rs. \,6$ per $kg.$ Find the average price of the mixture (in $Rs.$ per $kg$).

$200$ litres of mixture contains $15 \%$ water and the rest is milk. The amount of milk that must be added so that the resulting mixture contains $87.5 \%$ milk is..........$Litre$

$A$ tub contains a mixture of two liquids $P$ and $Q$ in the ratio of $4:1$. When $10 \, L$ of the mixture is taken out and $10 \, L$ of liquid $Q$ is poured into the tub,the ratio becomes $2:3$. How many litres of liquid $P$ were contained in the tub initially?