From March 10 to May 12 = 63 days.
63 days / 365 days = 0.1726 years.
Simple interest formula = Interest = Principal X interest rate x time
Interest = 15305.50 - 15000 = 305.50
Principal = 15000
Time = 0.1726 years
Replace the values into the formula:
305.50 = 15000 x Interest rate x 0.1726
Simplify:
305.50 = 2589.041 x interest rate
Solve for interest rate:
interest rate = 305.50 / 2589.041
interest rate = 0.11799 x 100
Rate = 11.80%