? ?
+   ?
-----
?


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)

• Are rot13(artngvir qvtvgf) allowed? – Rand al'Thor Jan 29 at 9:09
• @Randal'Thor No, they are not – yunzen Jan 29 at 9:09

Following in @jafe's footsteps:

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


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:

• 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. – yunzen Jan 29 at 9:18

Mixing bases is the first thing I can think of, e.g.:
$$10_2 + 1_2 = 3_{10}$$

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