STEP 1: Convert interest rate of 2% per 6 months into rate per year.
rate per year = rate per 6 month⋅2=2%⋅2=4%
STEP 2: Convert 9 months into years.
9 months =
9
12
years=0.75 years
STEP 3: Find principal by using the formula
I=P⋅i⋅t
, where I is interest, P is total principal, i is rate of interest per year, and t is total time in years.
In this example I = $15, i = 4% and t = 0.75 years, so
I
P
P
P
=P⋅i⋅t
=
I
i⋅t
=
15
0.04⋅0.75
=500