$A$ mixture of $30 \ L$ contains milk and water in the ratio of $7:3$. How much water should be added to it so that the ratio of milk and water becomes $3:7$? (in $L$)

Two varieties of sugar are mixed together in a certain ratio. The cost of the mixture per $kg$ is ₹ $0.50$ less than that of the superior and ₹ $0.75$ more than the inferior variety. The ratio in which the superior and inferior varieties of sugar have been mixed is

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

Two casks of $48$ $litres$ and $42$ $litres$ are filled with mixtures of wine and water,the proportions in the two casks being respectively $13:7$ and $18:17$. If the contents of the two casks are mixed,and $20$ $litres$ of water is added to the whole,what will be the proportion of wine to water in the resultant mixture?

$A$ person covers a distance of $100\, km$ in $10\, hours$,partly by walking at $7\, km/h$ and the rest by running at $12\, km/h$. Find out the distance covered in each part (in $km$).

$50 \, g$ of an alloy of gold and silver contains $80 \%$ gold (by weight). The quantity of gold,that is to be mixed with this alloy,so that it may contain $95 \%$ gold,is (in $g$)