SOLUTION STEPS:
  1. Total marbles = 25 + 63 = 88. This stays the same (marbles just move).
  2. After: Shaun has 70 more than Mark. Total = Mark + (Mark + 70) = 2 x Mark + 70 = 88.
  3. 2 x Mark = 88 - 70 = 18. Mark left = 9 marbles.