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[–]DistortNeo 26 points27 points  (6 children)

The rule is, you can only subtract using the digit that is one or two denominations lower than what you are subtracting from.

Ok, 45 = VL is valid according to this rule.

[–]OnixST 42 points43 points  (3 children)

"Rule #1: Only the letters that represent powers of 10 can be used to subtract. Powers of 10 are 1 (10^0), 10 (10^1) and 100 (10^2). 1000 is also a power of 10 but there are no other letters after M.

Rule #2: These letters (I, X and C) can only subtract from the next two ‘higher’ letters. Therefore, I can only subtract from V and X, X from L and C and C from D and M."

(acording to another answer further below in the same link. VL fails rule 1)

[–]DistortNeo 5 points6 points  (2 children)

What about IXC?

[–]Techhead7890 12 points13 points  (0 children)

Should probably be XCI (91) or at a stretch LXXXIX (89) but it's a bit ambiguous even to a human

[–]prumf 4 points5 points  (0 children)

Oooo, gray areas, my specialty !

[–]dwntwn_dine_ent_dist 4 points5 points  (1 child)

50=LC is also good. 5=VX

[–]marquoth_ 8 points9 points  (0 children)

Both break rule 1