15th Mexican Mathematical Olympiad Problems 2001

15th Mexican Mathematical Olympiad Problems 2001A1.  Find all 7-digit numbers which are multiples of 21 and which have each digit 3 or 7. A2.  Given some colored balls (at least three different colors) and at least three boxes. The balls are put into the boxes so that no box is empty and we cannot find three balls of different colors which are in three