$A$ 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 $4:3$?

How many $kg$ of salt at $42$ $P$ per $kg$ must a man mix with $25 \, kg$ of salt at $24$ $P$ per $kg$,so that he may,on selling the mixture at $40$ $P$ per $kg$,gain $25 \%$ on the outlay?

$A$ man has $60$ pens. He sells some of these at a profit of $12 \%$ and the rest at $8 \%$ loss. On the whole,he gets a profit of $11 \%$. How many pens were sold at $12 \%$ profit?

In a mixture of $75 \ L$,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 the milk to water $1:2$ will be (in $L$):

In a $120$ $litre$ mixture of milk and water,water is only $25 \%$. The milkman sold $20$ $litre$ of this mixture and then added $16.2$ $litre$ of pure milk and $3.8$ $litre$ of pure water to the remaining mixture. What is the percentage of water in the final mixture (in $\%$)?

$A$ $12 \text{ litres}$ 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?