Cheating on a string theory exam
Pick 5400 23-bit vectors (the answers you will write) and your friend picks the one that minimizes its Hamming distance to the true answer.<p><pre><code>    5400*23 + 5400*23*22/2 < 2**23
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so answering 21 questions with certainty is impossible. (Pick the true answers so as to maximize the Hamming distance between it and whatever answer you decide to write, regardless of the scheme used.)<p>And there is a size 4096 < 5400 covering code [1] so answering 20 question is possible (by picking the 23-bit vectors in that code).<p>[1] see n = 23, R = 3 here <a href="http://www.sztaki.hu/~keri/codes/2_tables.pdf" rel="nofollow">http://www.sztaki.hu/~keri/codes/2_tables.pdf</a><p>Edit: formatting