If on selling $12$ notebooks,a seller makes a profit equal to the selling price of $4$ notebooks,what is his percent profit?

If the selling price of $\frac{2}{3}$ of a certain quantity of milk is equal to the cost price of the whole quantity of milk,then what will be the gain percent in this transaction?

An article is sold at a profit of $20 \%$. If both the cost price and selling price are $Rs. 150$ less,the profit would be $5 \%$ more. Find the cost price.

$A$ shopkeeper professes to sell his goods at cost price but uses a $930 \text{ g}$ weight instead of $1 \text{ kg}$ weight. What will be the profit percentage of the shopkeeper (in $\%$)?

$A$ cycle dealer offers a discount of $10 \%$ and still makes a profit of $26 \%$. What does he pay for a cycle whose marked price is $₹ 840$? (in $₹$)

$A$ dishonest dealer professes to sell his goods at cost price but uses a weight of $960 \, g$ instead of a $1 \, kg$ weight. Find his gain $\%$. (in $\%$)