Assuming the 45 min classes were on the weekdays, this is the equation I would use:
60+45a=285
Where 60 is the Saturday class, 45a are the 45 minute weekday classes times a amount of classes and 285 is the total amount of minutes.
Also, here is how to solve the equation to find the number of weekday classes, which is 5.
60+45a=285
-60 -60
45a=225
----- -----
45 45
a=5