ELEMENTARY!
GAMES Big Book of Games
Stan Kurzban

Given the equation
                                 5 - 4 = IRON

What chemical element could replace the question mark in this equation?
                                 6 -  9 = ?

HINTS:
1. Number words
2. Roman numerals
3. Chemical symbols

Answer:

SULFUR

The first equation could be changed to
                FIVE - IV = FE (dropping the IV in FIVE)
                                            (Fe is the symbol for iron),       
 The second equation could be solved similarly:
               SIX -IX = S (the symbol for sulfur)