How many kilograms of firewood do you need to burn to get the same amount of heat as when burning 2 kg of coal?

The amount of heat that will be released during the complete combustion of coal:
Q = q1 * m1, q1 is the specific heat of combustion of coal (we take q1 = 29.3 * 10 ^ 6 J / kg), m1 is the mass of coal (m1 = 2 kg).
The amount of heat that will be released during the complete combustion of firewood:
Q = q2 * m2, q2 is the specific heat of combustion of firewood (we take for dry firewood q = 15 * 10 ^ 6 J / kg), m2 is the mass of firewood.
q1 * m1 = q2 * m2.
m2 = q1 * m1 / q2 = (29.3 * 10 ^ 6) * 2 / (15 * 10 ^ 6) = 3.91 kg.
Answer: You need to burn 3.91 kg of firewood.