What is the sum of X+XX+XXX+XXXX?
10.06.2025 09:16

Given the use of upper case, I personally would initially assume Roman numerals*, so the answer would be C (100 in present-day decimal notation)
as would be assuming that these are representations of conventional numbers, in which case the answer for number bases of five or more would be
and for binary would be
11010X
*Note that the abbreviation XL (for XXXX) appeared quite late in the Roman period, and never found universal favour
1234X
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However, X +X^2 +X^3 + X^4 is a valid algebraic answer,
What is the sum of X+XX+XXX+XXXX?
It’s a matter of interpretation.