Let A represent the total amount of money
r represent the registration fee.
There were 124 new members. Since 50 had their initial registration fee waived, 74 paid the registration fee.
---> Amount earned at registration: A = 74·r
Assuming that no-one dropped out, 6 months later:
Let m represent the monthly fee
---> Total amount earned for the first 6 months: A = 74·r + 124·m·6