A dishonest merchant sells goods at a 12.5% loss on the cost price, but uses 28 g weight instead of 36 g. What is his percentage profit or loss?

Answer: Option B

Let 36 gm weight cost price = Rs. 80
1 gm weight cost price = $$=\frac{80}{36}=\frac{20}{9}$$
28 gm weight selling price = Rs. 70
28 gm weight selling price $$=\frac{70}{28}=\frac{5}{2}$$
Profit $$=\frac{5}{2}-\frac{20}{9}=\frac{5}{18}$$
Profit % $$=\frac{5\times 9}{18\times 20}\times 100=\frac{25}{2}=12.5\%$$

$$\begin{array}{rl}& \mathbf{Alternate}\mathbf{solution}:\\ & 12.5\%=\frac{1}{8}\\ & \frac{\text{Selling price}}{\text{Cost price}}=\frac{7}{8}\times \frac{36}{28}=\frac{9}{8})1\text{Profit}\\ & \text{Profit }\%=\frac{1}{8}\times 100=12.5\%\end{array}$$