When fully immersed in water, a glass sphere with a volume of 200 cm³ weighs three times less than in air

When fully immersed in water, a glass sphere with a volume of 200 cm³ weighs three times less than in air. Find the volume of the cavity inside the ball. Density of glass P st = 2.5 g / cm³, density of water P in = 1 g / cm³

The weight of the ball in water Pv is three times less than the weight of the ball in air P:
Pv = P / 3 (1);
3Pv = P (2).
The weight of the ball in water is less, because the buoyancy force of Archimedes acts on it:
Pv = P – Fa (3).
Substitute (3) into (2):
3 (P-Fa) = P (4);
2P = 3Fa (5).
Answer: Answer: 0.00008 m³ (80 cm³).