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 $30 \%$?

$A$ man had a certain amount with him. He spent $20\%$ of that to buy an article and $5\%$ of the remaining on transport. Then he gifted $Rs. 120$. If he is left with $Rs. 1,400$,the amount he spent on transport is (in $Rs.$)

An article is marked for sale at $Rs. 504$. The shopkeeper gives a discount of $5\%$ on the marked price and still earns a profit of $20\%$. What is the cost price of the article? (in $Rs.$)

Ram purchased a computer set for $Rs. 12500$ and spent $Rs. 300$ on transportation and $Rs. 800$ on installation. At what price should he sell it to earn an overall profit of $15\%$? (in $Rs.$)

On selling an article for $Rs. 170$,a shopkeeper loses $15\%$. In order to gain $20\%$,he must sell that article at rupees:

Some articles were bought at $6$ articles for $Rs. 5$ and sold at $5$ articles for $Rs. 6$. The gain percent is: (in $\%$)