$A$ mixture of $40$ $litres$ of milk and water contains $10 \%$ water. How much water must be added to make $20 \%$ water in the new mixture? (in $Liters$)

$A$ cylinder which has the capacity of $8 \text{ litres}$ contains a mixture of oxygen and nitrogen. The volume of oxygen in the cylinder is $16 \%$ 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 $9 \%$ of the total volume. Find the amount of mixture released each time (in litres).

In a mixture of $75 \, litres$,the ratio of milk to water is $2:1$. The amount of water to be further added to the mixture so as to make the ratio of milk to water $1:2$ will be (in $litres$):

$A$ mixture contains $80 \%$ acid and the rest water. What part of the mixture should be removed and replaced by the same amount of water to make the ratio of acid to water $4:3$?

$A$ shopkeeper has $50 \ kg$ of rice,a part of which he sells at $10 \%$ profit and the rest at $5 \%$ loss. He gains $7 \%$ on the whole. What is the quantity sold at $10 \%$ gain?

$A$ sum of ₹ $390$ was divided among $450$ boys and girls. Each girl gets $50$ paise and each boy gets one rupee. How many girls are there among them?