If by selling an article for Rs. 390 a shopkeeper gains 20%, then the cost is = ?

Answer: Option B

$$20\%=\frac{1\to \text{Profit}}{5\to \text{CP}}$$
Selling price = 6 units → 390
$$\begin{array}{rl}& \text{5 units }\to \frac{390\times 5}{6}\\ & =325\end{array}$$