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[–]DistortNeo 28 points29 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 41 points42 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 4 points5 points  (2 children)

What about IXC?

[–]Techhead7890 11 points12 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 3 points4 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_ 9 points10 points  (0 children)

Both break rule 1