The number of movies that are 2.5 hours long that Cora watched is 10 hours.
a + b = 30 equation 1
1.5a + 2.5b = 55 equation 2
Where:
a = movies that are 1.5 hours long
b = movies that are 2.5 hours long
Multiply equation 1 by 1.5
1.5a + 1.5b = 45 equation 3
Subtract equation 3 from 2
1b = 10 hours
To learn more about simultaneous equations, please check: https://brainly.com/question/25875552