  We provide a simple way for calculating the entropy of a Schwarzschild black
hole from the entropy of its Hawking radiation. To this end, we show that if a
thermodynamic system loses its energy only through the black body radiation,
its loss of entropy is always 3/4 of the entropy of the emitted radiation. This
proposition enables us to relate the entropy of an evaporating black hole to
the entropy of its Hawking radiation. Explicitly, by calculating the entropy of
the Hawking radiation emitted in the full period of evaporation of the black
hole, we find the Bekenstein-Hawking entropy of the initial black hole.
