If the profit per cent got on selling an article is numerically equal to its cost price in rupees and the selling price is Rs. 39, then cost price (in Rs.) will be:

Answer: Option D

SP = Rs. 39
CP = x(let)
Profit % = CP
$$\text{or,}\frac{39-x}{x}\times 100=x$$       $$\left[\%\text{profit}=\frac{\text{SP}-\text{CP}}{\text{CP}}\right]$$
$$\begin{array}{rl}& 3900-100x=x^2\\ & x^2+100-3900=0\end{array}$$  
$$x=30$$   (we cannot take negative value of x)