Volume of balloon = $\frac{4}{3} \pi r^3$ , Radius of balloon, r = 10 m V = $\frac{4}{3}$ × 3.14 × $(10)^3$ = 4186.7 $\mathrm{m}^3$ Mass of displaced air = 4186.7 $\mathrm{m}^3$ × 1.2 $\mathrm{kg\,m}^{-3}$ = 5024.04 kg Moles of gas present = $\frac{PV}{RT}$ = $\frac{1.66 \times 4186.7 \times 10^3}{0.083 \times 300}$ = 279.11 × $10^3$ moles Mass of helium present = 279.11 × $10^3$ × 4 = 1116.44 × $10^3$ g = 1116.44 kg Mass of filled balloon = 100 + 1116.44 = 1216.44 kg Payload = Mass of displaced air – Mass of balloon = 5024.04 – 1216.44 = 3807.6 kg