According to Poiseuille's Law, the volume of blood per unit time that flows past a given position in a blood vessel is proportional to the fourth power of the radius of the vessel. In other words, $F = kr^4$, where $F$ is the flux, $r$ is the radius, and $k$ is a constant. If $r$ changes by 3 percent, use differentials to approximate the percentage change in $F$.

Answer: %