In what ratio must sugar at $Rs. 2$ per $kg$ be mixed with sugar at $Rs. 3.50$ per $kg$ so that the mixture is worth $Rs. 2.50$ per $kg$?

$I$ have $100$ books of English. $I$ sold some of these at a profit of $12 \%$ and the rest at $8 \%$ loss. On the whole,$I$ got a profit of $11 \%$. How many books were sold at $12 \%$ profit by me?

$729 \ ml$ of a mixture contains milk and water in the ratio $7:2$. How much more water is to be added to get a new mixture containing milk and water in the ratio $7:3$? (in $ml$)

$A$ merchant has $1000 \ kg$ of rice,part of which he sells at $8 \%$ profit and the rest at $16 \%$ profit. He gains $14 \%$ on the whole. The quantity sold at $16 \%$ profit is (in $kg$):

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?

$A$ merchant has $1000 \ kg$ of sugar,part of which he sells at $8 \%$ profit and the rest at $18 \%$ profit. If he gains $14 \%$ on the whole,then the quantity sold at $18 \%$ profit is.......$kg$.