Question

A girl sold her pen for Rs. $39$ and got a percentage of profit numerically equal to the cost price. The cost price of that pen is :

• A. $130$
• B. $90$
• C. $60$
• D. $30$

Answer 1

The correct option is D $30$

Let C.P. was Rs. $x$, then $\%$ profit must be $x\%$
Selling price $=$ Rs. $39$
Therefore, cost price $+$ profit $=39$
$x+x\%$ of $x=39$
$x+\left(x^2\right)/100=39$
$100x+x^2=3900$
$x^2+100x-3900=0$
$x^2+130x-30x-3900=0$
$x=-130,30$
Cost price cannot be negative, so cost price was Rs. $30$.