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 ?

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

Alternate :
$$\eqalign{ & {\text{Loss}}\% {\text{ = }}\frac{{x \times y}}{{100}}\% \cr & {\text{Loss}}\% {\text{ = }}\frac{{10 \times 10}}{{100}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = 1\% \cr} $$