Answer
a. The buoyant force must be equal and opposite to gravity, 20000 N. The buoyant force is also equal to the weight of the displaced air, so the volume of displaced air weighs 20000 N.
b. 20000 N is the buoyant force, so 20000 N = (balloon volume)(weight density of air). On page 271, the latter number is given as $12 \frac{N}{m^{3}}$.
Solve for the volume of the balloon, $\frac{20000 N}{12 N/m^{3}} = 1700 m^{3}$.
Work Step by Step
This is discussed on page 271.
