Heating the water in the tank takes 20% of the energy released during the combustion of firewood in the stove

Heating the water in the tank takes 20% of the energy released during the combustion of firewood in the stove. How much wood must be burned in the oven to heat 200 liters of water from 20 to 80 degrees?

Efficiency = 20%.

V = 200 L = 0.2 m ^ 3.

t1 = 20 ° C.

t2 = 80 ° C.

C = 4200 J / kg * ° C.

ρ = 1000 kg / m ^ 3.

qdr = 1 * 10 ^ 7 J / kg.

mdr -?

Efficiency = Qw * 100% / Qdr.

Qdr = qdr * mdr.

Qw = C * m * (t2 – t1) = C * ρ * V * (t2 – t1).

Efficiency = С * ρ * V * (t2 – t1) * 100% / qdr * mdr.

mdr = C * ρ * V * (t2 – t1) * 100% / qdr * efficiency.

mdr = 4200 J / kg * ° C * 1000 kg / m ^ 3 * 0.2 m ^ 3 * (80 ° C – 20 ° C) * 100% / 1 * 10 ^ 7 J / kg * 20% = 25 , 2 kg.

Answer: to heat the water, you need to burn mdr = 25.2 kg of firewood.