Find the simple interest on an investment of $800 at a rate of 2.5% per year after 9 months.

I = PRT

I = 800 * 0.025 * 0.75

To find the simple interest on an investment, you can use the formula:

Interest = Principal * Rate * Time

Where:
- Principal is the initial amount invested ($800 in this case)
- Rate is the interest rate (2.5% per year, which needs to be converted to a decimal by dividing it by 100)
- Time is the duration of the investment (9 months, which needs to be converted to a fraction of a year by dividing it by 12)

Let's calculate it step by step:

1. Convert the interest rate to a decimal:
2.5% ÷ 100 = 0.025

2. Convert the time to a fraction of a year:
9 months ÷ 12 = 0.75 years

3. Plug the values into the formula:
Interest = $800 * 0.025 * 0.75

4. Calculate the interest:
Interest = $15

Therefore, the simple interest on the investment of $800 at a rate of 2.5% per year after 9 months is $15.