I=$54, P=$900, t. = 18 months

Wait what are trying to answer here

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%