At what annual interest rate musts $2500 be invested for three years in order to yield $412.50 in simple interest? Do you not round the decimals to percent, change the decimals to percent and write it as a fraction.

Simple interest means the interest money does not get added to the initial investment (principal) so you don't earn additional interest money on the interest you already earn.

Therefore to calculate the interest earned in one year, you multiply the initial investment or principal amount of money by the rate of interest per year. To get the interest for 3 years, just multiply by 3. In this example, you are already given the interest
amount and asked to find the interest rate. in real life, it is usually the other way around. But in any case, the formula is the same.

Interest = Principal x Interest rate x Time

Because the interest rate is annual (per year), the time must also be expressed in years.

What we don't know is the interest rate so we will call it R.

So you get 412.50 = 2500. x R x 3 and you solve for R

R = .055 = 5.5% = 55/1000