Well, we can eliminate notations like XL, IV, IX, etc because, as noted in the article, those were not in common usage until much later. Even if we don't, then the first step is to expand these values, much like you would with fractions (i.e., convert to lowest common denominator).
Subtraction is as easy as addition (unless you go into negative numbers). The stategy is to walk the subtrahend left-to-right, eliminating from both as you go. If you run into a value that you don't have, then you expand the minuend from the right until you do.
For your "difficult" example of 42 - 13:
XXXXII - XIII
1 Eliminate an X from both.
Now you have XXXII - III.
2 Eliminate I from both
Now you have XXXI - II. Repeating (2) gives you XXX - I.
3 Since there is no I to eliminate, expand the minuend from the right to create one.
Now you have XXVV - I, repeat since you still don't have a I, repeat (3) which gives you XXVIIIII - I
Eliminate the from I both, giving the final result: XXVIIII.
Optional, you can "reduce" this to XXVIV, like you would if you had a fraction, like 3/9, but as stated in the article, this notation was not common place in ancient Rome.
Subtraction is as easy as addition (unless you go into negative numbers). The stategy is to walk the subtrahend left-to-right, eliminating from both as you go. If you run into a value that you don't have, then you expand the minuend from the right until you do.
For your "difficult" example of 42 - 13:
XXXXII - XIII
1 Eliminate an X from both.
Now you have XXXII - III.
2 Eliminate I from both
Now you have XXXI - II. Repeating (2) gives you XXX - I.
3 Since there is no I to eliminate, expand the minuend from the right to create one.
Now you have XXVV - I, repeat since you still don't have a I, repeat (3) which gives you XXVIIIII - I
Eliminate the from I both, giving the final result: XXVIIII.
Optional, you can "reduce" this to XXVIV, like you would if you had a fraction, like 3/9, but as stated in the article, this notation was not common place in ancient Rome.