I=1 , V=5, X=10,  L=50,  C=100,  D=500,  M=1000 

There are three basic principles for reading and writing Roman numerals:

1. A letter repeats its value that many times. A letter can only be repeated three times. 

2. If one or more letters are placed after another letter of greater value, add that amount. 

3. If a letter is placed before another letter of greater value, subtract that amount. 

Several rules apply for subtracting amounts from Roman numerals:a. 

a. Only subtract powers of ten (I, X, or C, but not V or L)
b. Only subtract one number from another.
c. Do not subtract a number from one that is more than 10 times greater


XX  That repeats X=10 twice, so by rule 1, that is 10+10 or 20

XXV  That has a V=5 after XX, so by rule 2, we add that amount 20+5=25  

XXII  That has a II=2 after XX, so by rule 2, we add that amount 20+2=22

XXIX  That has IX after XX, and the I before the X in IX means that by rule 3,
      we subtract 1 from the X=10 and get 9, so we add 20+9 = 29

Edwin