by Andrew Skidmore
Published December 19 2010 (link)
Finley is excited about Christmas. At school, he has made a card showing a triangular-shaped Christmas tree with a fairy on top. With this in mind, please decipher XMAS + TREE = FAIRY, where different letters stand consistently for different digits, and where TREE is a triangular number (ie, is the sum of some consecutive integers from 1 upwards).
What number is XMAS?