How much water can be heated from 20 ° to 70 °, using the heat released during the complete combustion of 0.42 kg of firewood.

t1 = 20 ° C.
t2 = 70 ° C.
C = 4200 J / kg * ° С.
md = 0.42 kg.
λ = 10,000,000 J / kg.
mv -?
When firewood is burned, the released amount of heat energy is used to heat the water.
When burning firewood, the released amount of heat is expressed by the formula: Qd = λ * md, where λ is the specific heat of combustion of firewood, md is the mass of firewood that burned.
To heat water, the required amount of heat Q is expressed by the formula: Qw = C * mw * (t2 – t1), where C is the specific heat capacity of water, mw is the mass of water that has been heated, t2, t1 are the final and initial water temperatures.
Qd = Qw.
λ * md = C * mw * (t2 – t1).
mv = λ * md / C * (t2 – t1).
mw = 10,000,000 J / kg * 0.42 kg / 4200 J / kg * ° С * (70 ° С – 20 ° С) = 20 kg.
Answer: wood can be used to heat mw = 20 kg of water.