By selling an article for $Rs. 21$,a man incurs a loss such that the percentage loss is equal to the cost price. The cost price of the article is (in $Rs.$):

$A$ shopkeeper labeled the price of his articles in order to earn a profit of $30 \%$ on the cost price. He,then,sold the articles by offering a discount of $10 \%$ on the labelled price. What is the actual rate of profit he earned in the deal? (in $\%$)

The cost price of an article is $80 \%$ of its marked price for sale. How much percent does the tradesman gain after allowing a discount of $12 \%$?

The cost price of $12$ tables is equal to the selling price of $16$ tables. The loss percent is (in $\%$)

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

The percentage profit earned by selling an article for $Rs. 1920$ is equal to the percentage loss incurred by selling the same article for $Rs. 1280$. At what price should the article be sold to make $25 \%$ profit? (in $Rs.$)