If the ratio of the cost price and selling price of an article is $20: 21$,then what will be the profit percent (in $\%$)?

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

Tarun bought a $TV$ with $20 \%$ discount on the labelled price. Had he bought it with $25 \%$ discount,he would have saved $₹ 500$. At what price did he buy the $TV$?

$A$ white goods dealer pays $10 \%$ custom duty on an iPhone that costs $Rs. 25000$ in $UK$. For how much should he mark it,if he desires to make a profit of $20 \%$ after giving a discount of $25 \%$ to the buyer? (in $Rs$)

$A$ buys an article and sells it to $B$ at a profit of $10 \%$. $B$ sells it to $C$ gaining $20 \%$. If $C$ pays $₹ 924$,what amount did $A$ pay? (in $₹$)

$A$ man bought $15$ mangoes for a rupee. How many mangoes were sold for a rupee so that there is a loss of $25 \% ?$