Let
x--------> the number of sessions
y-------> the total payment
we know that
<u>without membership:</u>
-------> equation A
<u>with membership:</u>
-------> equation B
using a graph tool------> resolve the system of equations
the solution is the intersection both graphs
see the attached figure
the solution is the point 
that means
For 
the cost is
in both forms
so
to justify buying the membership

therefore
<u>the answer is</u>
The equation to determine how many sessions you must use the pool to justify buying the membership is
