If $x$ is $90 \%$ of $y,$ then what per cent of $x$ is $y$?

If an electricity bill is paid before the due date,one gets a reduction of $4 \%$ on the amount of the bill. By paying the bill before the due date,a person got a reduction of $₹ 13$. The amount of his electricity bill was:

$45 \%$ of $1500 + 35 \%$ of $1700 = ? \%$ of $3175$

The number of students in a class is increased by $20 \%$ and the number now becomes $66$. Initially,the number was:

$A$ jar contains $10$ red marbles and $30$ green ones. How many red marbles must be added to the jar so that $60 \%$ of the marbles will be red?

Out of $450$ students of a school,$325$ play football,$175$ play cricket and $50$ neither play football nor cricket. How many students play both football and cricket?