 D, L, or V may each only appear at most one time in the string
 M, C, X, or I may appear no more that three times consecutively in the string
 The following pairs of letters are invalid in all cases: IL, IC, ID, IM, XD, XM, VX, VL, VC, VD, VM, LC, LD, LM, DM.
 Once a letter has been subtracted from, neither it nor its "5 counterpart" may appear again in the string  so neither X nor V can follow IX, neither C nor L may follow XC, and neither M nor D may follow CM.
 Once a letter has been used as a subtraction modifier, it cannot appear again in the string  so C cannot follow CD or CM, X cannot follow XL or XC, and I cannot follow IV or IX.
 Once I, X, or C (or their "5counterparts" V, L, and D) appears in a string, the I, X, or C cannot subsequently be used as subtraction modifiers  so IV or IX cannot follow I or V, XL or XC cannot follow X or L, and CD or CM cannot follow C or D.
Example: What is the Hindu Arabic equivalent of XCII?
X  10
C  100
II  2
X before C implies 100  10
Hence 90 + 2
Answer: 92
Example: What is the Hindu Arabic equivalent of VD?
Answer: Not valid
More examples:
Numeral  Roman  Numeral  Roman 
150  CL  200  CC 
300  CCC  400  CD 
600  DC  700  DCC 
800  DCCC  900  CM 
1000  M  1600  MDC 
1700  MDCC  1900  MCM 
Directions: Write the corresponding Hindu Arabic numbers for the following. Also write at least 10 examples of your own.
