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

Answer 1

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

Answer 2

I = PRT

80 = 1500 * R * 0.75

80 = 1125R

80/1125 = R

0.071 = R

7.1% = R