His monthly payment should be $ 4.400639...
Explanation
Monthly payment formula is like...
, where M is the monthly payment, P is the principal amount, r is the monthly interest rate in decimal and n is the total number of months.
Here given that, P = $ 48
r = 18% annually =
% monthly = 1.5% monthly = 0.015
n = 1 year = 12 months
So, plugging these values into the above formula, we will get...

So, his monthly payment should be $ 4.400639...