Draw a bar model: Total marbles = 88 stays constant (marbles only change hands).
After: Mark = 1 unit, Shaun = 1 unit + 70. Total = 2 units + 70 = 88.
1 unit = 9 = Mark left.AGS Rule Box
| Total marbles | = | 25 + 63 = 88 |
| 2 units | = | 88 - 70 = 18 |
| Mark left (1 unit) | = | 18 / 2 = 9 |
Final Answer: 9