Answer:
5m.
Step-by-step explanation:
The virtual image of the ball is exactly 5m behind the mirror, and the virtual ball behaves exactly like the real ball except that it is located behind the mirror (and it is not a real ball). The two balls, the real and the virtual, hit each other when they both reach the mirror, and if the real ball travels 5 meters to get to the mirror, so must the virtual ball.
Hence, the virtual ball travels 5 meters before it hits the real ball.