Answer:
Step-by-step explanation
We get three linear equations from the information given, where
p= number of premium tickets
d = number of deluxe tickets
r = number of regular tickets:

and the applying third r=118+d, we get


Now we get from the upper one
p=115-2d
solving the another equation gives us
6*115-12d+6d=600,
hence d=15
and by replacing
p=115-2*15=85.
85 premium tickets were sold