$300\, gm$ of salt solution has $40 \%$ salt in it. How much salt should be added to make it $50 \%$ in the solution? (in $gm$)

In a laboratory,two bottles contain a mixture of acid and water in the ratio $2:5$ in the first bottle and $7:3$ in the second. The ratio in which the contents of these two bottles should be mixed such that the new mixture has acid and water in the ratio $2:3$ is

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 $5:4$?

$A$ container contains $50 \, \text{litres}$ of milk. From this container $5 \, \text{litres}$ of milk was taken out and replaced by water. This process was repeated further two times. How much milk is now contained by the container? (in $\text{litres}$)

$A$ trader has $50 \ kg$ of sugar,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 and $5 \%$ loss? (in $kg$)

Gold is $19$ times as heavy as water and copper is $9$ times as heavy as water. In what ratio should these metals be mixed so that the mixture may be $15$ times as heavy as water?