If the cost price of 10 articles is equal to the selling price of 7 articles, then the gain or loss percent is = ?

Answer: Option B

According to the question,
10 Cost Price = 7 Selling Price
$$\begin{array}{rl}& \frac{\text{CP}}{\text{SP}}=\frac{7}{10}⟩\text{3 unit profit}\\ & \text{Profit}\%=\frac{3}{7}\times 100\\ & =42\frac{6}{7}\%\text{ gain}\end{array}$$