When Mr. Teng retired on 1 January 2018, he put a sum of 500 at the start of each month for groceries. Denote the amount of money that Mr. Teng has at the time years by .
(i) The differential equation relating and can be written in the form of . State the values of and .
(ii) Solve the differential equation and find the amount of money that Mr. Teng has after 15 months.
(iii) In which month will Mr. Teng no longer be able to withdraw the full $400?