A clock was sold for Rs. 144. If the percentage of profit was numerically equal to the cost price, the cost of the clock was -

Answer: Option B

Let,
C.P. = Rs. x,
Profit% = x% and
S.P. = Rs. 144
$$\begin{array}{rl}& \therefore x=\left[\frac{100}{\left(100+x\right)}\times 144\right]\\ & \Rightarrow x^2+100x=14400\\ & \Rightarrow x^2+100x-14400=0\\ & \Rightarrow x^2+180x-80x-14400=0\\ & \Rightarrow \left(x+180\right)\left(x-80\right)=0\\ & \Rightarrow x=80\end{array}$$