How much dry wood needs to be burned to heat 100 liters of water from 20 degrees

How much dry wood needs to be burned to heat 100 liters of water from 20 degrees Celsius to 100 degrees Celsius if the efficiency of the system heater is 50%.

Initial data: V (volume of heated water) = 100 l = 0.1 m ^ 3; t0 (temperature before heating) = 20 ºС; t (final temperature) = 100 ºС; η (heater efficiency) = 50% or 0.5.

Reference data: ρ (water density) = 1000 kg / m ^ 3; C1 (specific heat of water) = 460 J / (kg * K); q (specific heat of combustion of dry wood) = 10 ^ 7 J / kg.

Efficiency: η = С * ρ * V * (t – t0) / (q * m).

m = C * ρ * V * (t – t0) / (η * q) = 4200 * 1000 * 0.1 * (100 – 20) / (0.5 * 10 ^ 7) = 6.72 kg.

Answer: You need to burn 6.72 kg of dry firewood.