Bob borrowed $1500 from his fathers or 9 months and paid back $1580. What is the simple interest rate?

80/1500 * 12/9 * 100 = 7.1%

I = PRT

80 = 1500 * R * 0.75

80 = 1125R

80/1125 = R

0.071 = R

7.1% = R

To find the simple interest rate, we need to use the formula:

Simple Interest = Principal * Interest Rate * Time

Where:
Principal is the amount borrowed ($1500),
Interest Rate is the rate at which the interest is charged (unknown),
Time is the duration for which the money was borrowed (9 months), and
Simple Interest is the amount paid back ($1580 - $1500 = $80).

Let's rearrange the formula to solve for the interest rate:

Interest Rate = Simple Interest / (Principal * Time)

Plugging in the values, we have:

Interest Rate = $80 / ($1500 * 9 months)

To get the interest rate, we divide $80 by $1500 and multiply by 1/9 to account for the 9 months:

Interest Rate = ($80 / $1500) * (1/9)

Calculating this, we find:

Interest Rate ≈ 0.0055556 or 0.556% (rounded to three decimal places)

Therefore, the simple interest rate is approximately 0.556%.