Answer
$\sqrt[4] (20x^{3})$
Work Step by Step
The product rule holds that $\sqrt[n] a\times\sqrt[n] b=\sqrt[n] (ab)$ (where $\sqrt[n] a$ and $\sqrt[n] b$ are real numbers).
Therefore, $\sqrt[4] 4x^{3}\times\sqrt[4] 5=\sqrt[4] (4x^{3}\times5)=\sqrt[4] (20x^{3})$