The main mathematical operation to perform when calculating with the simple interest formula is

The main mathematical operation to perform when calculating with the simple interest formula is multiplication.

Here are the steps:

1. Identify the principal amount (P), which is the initial sum of money.
2. Determine the annual interest rate (r), expressed as a decimal.
3. Determine the time period (t) for which the interest is calculated, usually in years.
4. Use the simple interest formula: $I = P \times r \times t$.

Where:

• $I$ is the interest.
• $P$ is the principal amount.
• $r$ is the annual interest rate.
• $t$ is the time period in years.

5. Multiply the principal amount (P) by the annual interest rate (r).
6. Multiply the result by the time period (t).

The final result is the simple interest earned over the specified time period.