Answer
Please see the work below.
Work Step by Step
We know that
$v=\frac{d_x}{d_t}=4bt^3$
and we also know that the average velocity is
$v=\frac{\Delta x}{\Delta t}$
$v=\frac{bt^4}{t}=bt^3$
Hence, the average velocity is one fourth of the instantaneous velocity.
