A tradesman marks his goods 10% above his cost price. If he allows his customers 10% discount on the marked price. How much profit or loss does he make, if any ?

Answer: Option B

Let cost price of goods = Rs 100
Market price of goods
$$\begin{array}{rl}& =110\%\text{ of }100\\ & =\frac{110}{100}\times 100\\ & \text{ = Rs}\text{. 110}\end{array}$$
After discount selling price of goods
$$\begin{array}{rl}& =90\%\text{ of }110\\ & =\frac{90}{100}\times 110\\ & =\text{Rs}\text{. 99}\end{array}$$
Loss = 100 - 99 = Rs. 1
$$\begin{array}{rl}& \text{Loss }\%\text{ = }\frac{1}{100}\times 100\\ & =1\%\end{array}$$

Alternate :
$$\begin{array}{rl}& \text{Loss}\%\text{ = }\frac{x\times y}{100}\%\\ & \text{Loss}\%\text{ = }\frac{10\times 10}{100}\\ & =1\%\end{array}$$