The number Garry left was greater than 3 but less than 9, so it must be one of the following: 4, 6, or 8. Additionally, the number must be even, which means it could be 4, 6, or 8.
The key clue is that the number, when reversed, should look like another number. Reversing 6 gives 9, which satisfies this condition. Therefore, the number Garry left is 6