This joke relies on the fact that mathematical expressions, just as expressions in natural languages, may have multiple meanings. Like other?puns, the humor is derived from an ambiguous meaning; in this case, the expression?10?in the?binary numeral system?is equal to the decimal number?two.

Another pun using different?radices, sometimes attributed to computer scientists, asks:

The humor lies in the similarity of the abbreviation for October/Octal?and December/Decimal, and the coincidence that the two representations equal the same amount (31 Octal is 3*8 + 1 = 2*10 + 5 = 25 Decimal).

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