Roman Numerals

During our morning maths hotspot we had to answer 20 mathematical questions from a range of strands, as well as numeracy.
One of the questions was about Roman Numerals. We had to write 475 using roman numerals.

Monique and Tiaana came up with two different answers:

CCCCLXXV

and

CDLXXV

Do you which one is correct?



We were a little confused, but with a bit of research we came across this website that helped explain when to use the subtracting rule.

Purple Math - Roman Numerals
The next number is 4, which is 1 + 1 + 1 + 1, but using four I's for "4" isn't allowed (because we can't do four of the same numeral in a row). If we were dealing with money, we could pay four dollars with four singles (which is 1 + 1 + 1 + 1 = 4) or we could pay with a five and get a single in change (which is 5 – 1 = 4). In Roman numerals, we "subtract" the 1 from the 5 by putting an I in front of a V: IV is "four" in the sense of "one, from five, leaves four".

IV = (5 – 1) = 4

In Roman numerals, we can do this subtractive thing with Is, Xs, and Cs (that is, with ones, tens, and hundreds) but not with Vs, Ls, or Ds (that is, not with fives, fifties, or five hundreds). 

So we now know that the correct answer is: CDLXXV

Can you give us some reasons for learning about roman numerals?