Answer: To find the annual rate of interest for simple interest, we can use the formula:
Simple Interest (SI) = Principal (P) * Rate (R) * Time (T) / 100.
Given that the principal (P) is $3100, the simple interest earned is $1052.8, and the time (T) is 4 years, we can solve for the rate (R):
SI = P * R * T / 100
1052.8 = 3100 * R * 4 / 100
Now, isolate the rate (R) by dividing both sides by (3100 * 4 / 100):
R = 1052.8 / (3100 * 4 / 100)
Now, calculate the rate (R):
R = 1052.8 / (3100 * 4 / 100)
R = 1052.8 / 124
Finally, calculate the rate (R) to find the annual interest rate:
R ≈ 8.50.
So, the annual rate of interest required for the investment of $3100 to earn $1052.8 in simple interest over four years is approximately 8.50%.