What is the number of days it will take for the plant to grow to the maximum height?

4 6 7 10

4

$y=4x-\frac{x^2}{2}$ $\frac{dy}{dx}=4-x=0⇒x=4$ $\frac{d^2y}{dx^2}=-1<0$ (Point of maxima) at $x=4$ days plant grows to max. height