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?

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 % $$ = \frac{{5 \times 9}}{{18 \times 20}} \times 100 = \frac{{25}}{2} = 12.5\% $$

$$\eqalign{ & {\bf{Alternate}}\,{\bf{solution:}} \cr & 12.5\% = \frac{1}{8} \cr & \frac{{{\text{Selling price}}}}{{{\text{Cost price}}}} = \frac{7}{8} \times \frac{{36}}{{28}} = \left. {\frac{9}{8}} \right)1\,{\text{Profit}} \cr & {\text{Profit }}\% = \frac{1}{8} \times 100 = 12.5\% \cr} $$