How much white cast iron can be melted in a furnace with an efficiency of 18% if 3.0 tons of A-I coal are consumed?

How much white cast iron can be melted in a furnace with an efficiency of 18% if 3.0 tons of A-I coal are consumed? The initial temperature of the cast iron is 293 K.

Data: η (efficiency) = 18% = 0.18; m1 (mass of coal A-I, anthracite) = 3 t = 3000 kg; t (initial temp.) = 293 К = 20 ºС.
Constants: tmelt (melting temperature) = 1200 ºС; q (beats heat of combustion of anthracite) = 3 * 10 ^ 7 J / kg; C (specific heat capacity) = 540 J / (kg * K); λ (specific heat of fusion of white cast iron) = 1.4 * 10 ^ 5 J / kg.
η = (C * m2 * (tmelt – t) + λ * m2) / (q * m1).
η * q * m1 = m2 * (С * (tpl – t) + λ).
m2 = η * q * m1 / (С * (tmelt – t) + λ) = 0.18 * 3 * 10 ^ 7 * 3000 / (540 * (1200 – 20) + 1.4 * 10 ^ 5) = 20844 kg.