Question

A trader bought a pen for Rs. x and sold it for RS.16. If his loss was $x\%$, find the cost price of the pen.

Answer 1

$L\%=\frac{L}{C.P}\times 100$
$L\%=\frac{C.P-S.P}{C.P}\times 100$
$x=\frac{x-16}{x}\times 100$
$x^2=100x-1600$
$x^2-100x+1600=0$
$x^2-80x-20x+1600=0$
$x(x-80)-20(x-80)=0$
$x=20,80$.