The simple interest formula calculates the interest earned or paid at the end of the time period.
The formula for simple interest is: \[ I = P \times r \times t \]
where:
- \( I \) is the interest,
- \( P \) is the principal amount (the initial amount of money),
- \( r \) is the annual interest rate (in decimal),
- \( t \) is the time the money is invested or borrowed (in years).
So, the correct answer is: the interest earned or paid at the end of the time period.