If the cost price of $12$ pens is equal to the selling price of $8$ pens,find the gain percent.

By selling $100$ oranges,a vendor gains the selling price of $20$ oranges. His gain percent is

$A$ man buys $300$ dozen of bananas for $Rs. 3,600$. He sold $1/3$ of the stock at a loss of $10\%$ and the remaining $2/3$ of the stock at a gain of $10\%$. Find his overall percent profit or loss.

By selling $80$ ball pens for $₹ 140$,a retailer loses $30 \%$. How many ball pens should he sell for $₹ 104$ so as to make a profit of $30 \% ?$

Vijay sold a watch at a gain of $5 \%$. Had he sold it for $Rs. 72$ more,he would have gained $13 \%$. Find out the cost price of the watch (in $Rs.$).

$A$ shopkeeper allows $23 \%$ commission on his advertised price and still makes a profit of $10 \%$. If he gains $₹ 56$ on one item,what is the advertised price of the item in $₹$?