Help ! I dont understand this Thermodynamics problem.?

Question

Here it is:
A spherical balloon has a diameter of 10ft. The average specific voume of the air inside is 15.1 ft^3/lb. Determine the weight of the air, in Lbf, at a location where g=31.0 ft/s^2.

Explanations would be helpful thanks :D !

Answer 1

Note: Specific volume is the inverse of density. Also, you should assume that the specific volume given is at sea level, otherwise you wouldn't need the gravity.

Step 1: Find volume of balloon:
V = 4/3πr^3 = 4/3 π * 5^3 = 523.6 ft^3.

Step 2: Find density - assume this is at normal gravity
d = 1/specific volume = 1/15.1 = 0.066225 lb/ft^3

Step 3: Find normal weight of air at sea level:
weight = V * d = 523.6 ft^3 * 0.066225 lb/ft^3 = 34.67541 lb

Step 4: Adjust for lower gravity.
Gravity at sea level (g): 32.17 ft/s²
Gravity given (g1): 31.0 ft/s²
Weight of air at sea level * g1/g = 34.67541 lb * 31.0 / 32.17 = 33.4 lb. (solution!)