The "interest" is always the annual rate, that is, 4% for a year.
The percent of interest you earned is (180/1500) = 0.12 = 12% .
Since you got 4% per year, your money was in the bank for
(12 %) x (1 year / 4%)
= (12 x 1 / 4) (% / %) (year) = 3 years .
