$A$ 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 $4:3$?

Pure milk costs $16 / L$. After adding water,the milkman sells the mixture at $15 / L$ and thereby makes a profit of $25 \%$. In what respective ratio does he mix milk with water?

Two vessels contain milk and water in the ratio $3:2$ and $7:3$. Find the ratio in which the contents of the two vessels have to be mixed to get a new mixture in which the ratio of milk and water is $2:1$.

The ratio of milk and water in $66$ $litres$ of adulterated milk is $5:1$. Water is added to it to make the ratio $5:3$. The quantity of water added is (in $litres$):

Zinc and copper are in the ratio $5:3$ in $400 \, g$ of an alloy. How much copper (in $g$) should be added to make the ratio $5:4$?

How many kg of sugar costing ₹ $11.70/kg$ must be mixed with $24$ $kg$ of sugar costing ₹ $8.1/kg$ so that there may be a gain of $15 \%$ by selling the mixture at ₹ $9/kg$? (in $kg$)