Simple interest gives us how much return our money gets after a period. There are certain data we need to properly calculate it.
The money invested is the Principal, P
The rate of return is R
The period for which it is invested is T
SIMPLE INTEREST FORMULA

In our case, we have:
P =$2500
R = 13.25% = 0.1325
T = 18 months = 1 year, 6 months = 1.5 years

Simple Interest = $496.875
Total amount after 18 months = $2,996.875