Answer:
Serena earns $7.50 per hour more than Gabe
Explanation:
Given
Serena:

Gabe
Earnings: $13.50 per hour ----- Missing Part of equation
Required
How much more does Serena earn per hour
For Serena
implies that She earns $21 per hour
So, to determine how much more She earns, we simply calculate the difference.
(per hour)
(per hour)
The difference implies that Serena earns $7.50 per hour more than Gabe