Answer
See answers.
Work Step by Step
a. Divide the classical expression for momentum by the relativistic momentum.
$$\frac{mv}{mv/(\sqrt{1-v^2/c^2})}=\sqrt{1-v^2/c^2}=\sqrt{1-0.15^2}=0.989$$
We see that the expression, mv, is too low by about $1.1\%$.
b. Repeat for v = 0.75c.
$$\frac{mv}{mv/(\sqrt{1-v^2/c^2})}=\sqrt{1-v^2/c^2}=\sqrt{1-0.75^2}=0.661$$
We see that the expression, mv, is too low by about $34\%$.