It is: CD = 500-100 = 400
CD = 400
CD = 400
XL=400 * * * * * No. XL is the Roman number 40. 400 is CD.
CD
If you mean CD plus DC then in Roman numerals they add up to M meaning 400 plus 600 = 1000
400 = CD
CD
CD = -100+500 = 400
CD-D
400 (500-100)
MCDXXXII (M=1000, CD=400, XXX=30, II = 2)