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If Friend A guesses 21, they lose because 21>19. If Friend B guesses 16 and Friend C guesses 12, then they've both theoretically arrived at viable answers, because 16 <19 and 12 <19. There is no 'extra credit' for Friend B, even though B's answer was closer to the target answer of 19.The question then becomes whether that energy associated with Bitcoin is put to good use.