4
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  ? ?
+   ?
-----
    ?

Replace every question mark (?) in this addition with a single digit so that the addition is valid and correct. The first (leftmost) question mark in the first (upper) line cannot be zero (0).

Credit:

(2000@G4G) Jaime Poniachik (1943–2011)

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  • $\begingroup$ Are rot13(artngvir qvtvgf) allowed? $\endgroup$ – Rand al'Thor Jan 29 at 9:09
  • $\begingroup$ @Randal'Thor No, they are not $\endgroup$ – yunzen Jan 29 at 9:09
9
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Following in @jafe's footsteps:

    I V          I X        X C
  +   I        +   I      +   X
 ------   or   -----  or  -----  etc.
      V            X          C

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6
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One possibility would be using

some other numbering system than Arabic numerals. The word "digit" can be used for other systems as well.

For example,

using rod numerals:

enter image description here

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  • $\begingroup$ Guvf vf gur evtug nccebnpu. V qba'g xabj, vs ebq ahzrenyf fubhyq pbhag nf n pbeerpg nafjre, ohg vg vf abg gur vagraqrq bar. $\endgroup$ – yunzen Jan 29 at 9:18
2
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Mixing bases is the first thing I can think of, e.g.:
$10_2 + 1_2 = 3_{10}$

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  • $\begingroup$ Interesting approach. rot13(Ab onfr punatr) $\endgroup$ – yunzen Jan 29 at 9:01

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