Changing Roman numerals to Hindu-Arabic

There are 27 patterns used to make up Roman numerals from 1 to 999. The patterns are put in order from largest to smallest as we read from left to right, which is true for Hindu-Arabic numerals as well.

1 through 9. With these patterns the first symbol is either I or V.
I = 1
II = 2
III = 3
IV = 4 (one before five)
V = 5
VI = 6
VII = 7
VIII = 8
IX = 9 (one before ten) 

10 through 90. With these patterns the first symbol is either X or L.
X = 10
XX = 20
XXX = 30
XL = 40 (ten before fifty)
L = 50
LX = 60
LXX = 70
LXXX = 80
XC = 90 (ten before hundred) 


100 through 900. With these patterns the first symbol is either C or D.
C = 100
CC = 200
CCC = 300
CD = 400 (hundred before five hundred)
D = 500
DC = 600
DCC = 700
DCC = 800
CM = 900 (hundred before thousand) 
M = 1,000, but another way to write numbers past 1,000, is to put a bar over the numbers.

For example,
__
IX = 9,000.

Change CDXCVII into Hindu-Arabic.

We start on the left and we find CDXCVII. CD = 400, the hundreds place

Then we have XCVII. XC = 90, the tens place.

VII = 7. Put them all together and we get CDXCVII = 497.

Change DIV into Hindu-Arabic.

D = 500, IV = 4, so this is 504. In Roman numerals they had no symbol for zero, so they just left that position blank.