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

If the cost price of $21$ watches is equal to the selling price of $18$ watches,then what would be the gain percent in this transaction?

On an article,the profit is $210 \%$ of the cost price. If the cost price increases by $40 \%$ but the selling price remains constant,approximately what percent of the selling price will be the profit?

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

$A$ shopkeeper bought $30\, Kg$ of wheat at the rate of $Rs.\, 45$ per $Kg$. He sold $40\%$ of the total quantity at the rate of $Rs.\, 50$ per $Kg$. Approximately,at what price per $Kg$ should he sell the remaining quantity to make $25\%$ overall profit? (in $Rs.$)

When Ram sold his book for $Rs. 255$,he made a loss of $15\%$. For how much should he have sold it if he wanted to make a profit of $5\%$?