Correct answer: 31.5 g
The heat lost by steam during condensation and cooling equals the heat gained by water to reach 80°C. Let m be the mass of condensed steam. Heat lost by steam = m × 540 + m × 1 × (100 - 80). Heat gained by water = 20 × 1 × (80 - 10). Equating, m × 540 + m × 20 = 20 × 70. Solving, m = 11.5 g. Total water mass = 20 + 11.5 = 31.5 g.