Derivative of $2x^2$ with respect to $5x^4$ is :

$\frac{1}{5x}$ $-\frac{1}{5x^2}$ $\frac{1}{5x^2}$ $\frac{1}{5x^3}$

$\frac{1}{5x^2}$

$a=2x^2$, $b=5x^4$ $\frac{da}{dx}=4x$, $\frac{db}{dx}=20x^3$ so $\frac{da/dx}{db/dx}=\frac{4x}{20x^3}$ so $\frac{da}{db}=\frac{1}{5x^2}$