Answer is: volume of the balloon is 487.485 liters.
n = 15 mol + 5 mol.
n = 20 mol; amount of substance.
p = 1.01 atm; pressure of the balloon.
T = 300 K; temperature.
R = 0.08206 L·atm/mol·K, universal gas constant.
Ideal gas law: p·V = n·R·T.
V = n · R · T ÷ p.
V = 20 mol · 0.08206 L·atm/mol·K · 300 K ÷ 1.01 atm.
V = 487.485 L; volume of the balloon.