Initial data: Q (the amount of heat that is required to be obtained when burning coal) = 8.1 * 10 ^ 7 J.
Reference data: q (specific heat of combustion of coal) = 27 MJ / kg = 27 * 10 ^ 6 J / kg.
We express the required mass of coal from the formula: Q = q * m, whence m = Q / q.
Calculation: m = 8.1 * 10 ^ 7 / (27 * 10 ^ 6) = 3 kg.
Answer: It is required to burn 3 kg of coal.
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