$A$ vendor bought toffees at $6$ for a rupee. How many for a rupee must he sell to gain $20 \%$?

Sanjay purchased a chair marked at $Rs. 800$ at $2$ successive discounts of $10 \%$ and $15 \%$ respectively. He spent $Rs. 28$ on transportation and sold the chair for $Rs. 800$. How much is his gain percentage (in $\%$)?

By selling $90$ pens for $Rs. 80$,a man loses $20\%$. What should be the selling price (in $Rs.$) of $90$ pens for $20\%$ profit?

$10 \%$ discount and then $20 \%$ discount in succession is equivalent to a total discount of $... \%$.

$A$ rice trader buys $16$ quintals of rice for $Rs. 5632$. $20 \%$ of the rice is lost in transportation. At what rate should he sell the remaining rice to earn a $25 \%$ profit in $Rs.$ per quintal?

$A$ retailer offers the following discount schemes for buyers on an article:
$I.$ Two successive discounts of $10 \%$
$II.$ $A$ discount of $12 \%$ followed by a discount of $8 \%$
$III.$ Successive discounts of $15 \%$ and $5 \%$
$IV.$ $A$ discount of $20 \%$
The selling price will be minimum under the scheme: