The simple interest formula is pretty straight forward. It's I = prt, where I is the interest earned, p is the initial amount invested, r is the interest rate expressed as a decimal, and t is time in years. We are looking for I; t = 3, p = 2500 and r = .015. Putting that altogether in our formula we have I = (2500)(3)(.015). That gives us 112.50 that was earned in interest.