Answer:
The lowest score 4th judge can give this dancer in order for her to move to the semi finals is 7.7.
Explanation:
Let x points be the score that 4th judge gave to the dancer.
The first three judges gave the dancer 8.9 points, 7.1 points and 8.3 points.
In total, the dancer got
points
The average score is

This average score must be at least of 8 points, so

The lowest score 4th judge can give this dancer in order for her to move to the semi finals is 7.7.