Aditya purchased a book with a $20 \%$ discount on the marked price. How much $Rs.$ did he pay if the marked price of the book was $Rs. 500$?

If a vendor sells a coconut at $Rs. 14.4$,he makes a $10 \%$ loss. If he wants to make a $25 \%$ profit,at what price (in $Rs.$) should he sell it?

$A$ sells an article to $B$ at a gain of $20 \%$,$B$ sells it to $C$ at a gain of $10 \%$,and $C$ sells it to $D$ at a gain of $12 \frac{1}{2} \%$. If $D$ pays $₹ 29.70$,then $A$ purchased the article for (in $₹$):

Mohan buys a watch for $Rs. 350$ and sells it for $Rs. 392$. Find out his percentage of profit.

The list price of a book is $Rs. 100$. $A$ dealer sells three such books for $Rs. 274.50$ after allowing a discount at a certain rate. Find the rate of discount in $\%$.

$A$ shopkeeper loses $7 \%$ by selling a cricket ball for $Rs. 31$. For how much should he sell the ball so as to gain $5 \%$? (in $Rs.$)