The most likely thing that has happened in this case is that a fee is paid for the registration.
The function that best models this problem is:
y = 45x + 15
Where,
x: number of months.
Y: payment of each month.
For the first month we have:
y = 45 (1) +15
y = 45 + 15
y = 60
answer:
the most likely reason for the bill being this amount is
registration fee.