Interest=Principal x Rate x Time (in years)
Interest=$54
Principal=$900
Convert 18 months to years by dividing by 12 (there are 12 months in a year) so
Time = 18/12=1.5
Plug into formula you get
54=(900)R(1.5)
54=1350R
Divide by 1350
.04=R
Annual Rate is 4%
