A company produces bikes at the rate of x bikes per day and its total cost function is $C(x) =x^3-60x^2+13x + 50.$ The optimal number of bikes produced per day at which the marginal cost is minimum is :

20

The correct answer is Option (3) → 20

Cost function = $C(x)=x^3-60x^2+13x+50$

$MC(x)=C'(x)=3x^2-120x+13$

for a quardritic function, $ax^2+bx+c$, the vertix occurs at 

$x=-\frac{b}{2a}=\frac{-120}{-2×3}=20$