A dealer sells his goods at a profit of 20%, but uses a weight of 800 g in place of a kg weight. Find his real gain percentage.

Answer: Option C

Selling price of 800 g goods with 20% profit $$=\frac{1000\left(100+20\right)}{100}=\text{Rs}\text{. }1200$$
Cost price of 800 g goods = Rs. 800
Gain = Selling price - Cost price = 1200 - 800 = 400
$$\begin{array}{rl}& \text{Gain }\%=\frac{400}{800}\times 100\%\\ & =\left(0.5\times 100\right)\%\\ & =50\%\end{array}$$
∴ His real gain percentage is 50%