
1) Firstly, let's express each plan by using linear equations.
Plan A:
5x+320
Note that the $320 payment is done once, then 5 per session.
Plan B
15x
2) To get to know when opting for plan A or Plan B is not relevant, we need to equate both equations and solve it for x (x stands for the number of sessions)

So if you attend 32 sessions then, doesn't really matter which plan is it.
3) Now, let's figure out which plan is more interesting to take:

So the plan B is cheaper to attend 1 session