Substitute digits for the letters to make the following relation true.
W O R L D
+ T R A D E
C E N T E R
[Note that the leftmost letter can't be zero in any word. Also, there
must be a one-to-one mapping between digits and letters.
e.g. if you substitute 3 for the letter W, no other letter can be 3
and all other W in the puzzle must be 3. ]