R = 0.05%/day
(R = 18%/year)
Equation:
r = (1/t)(A/P - 1)
Calculation:
Solving our equation:
r = (1/90)((418/400) - 1) = 0.0005
r = 0.0005
Converting r decimal to R a percentage
R = 0.0005 * 100 = 0.05%/day
Calculating the annual rate
0.05%/day × 360 days/year = 18%/year.
The interest rate required to get a total amount, principal plus interest, of $418.00 from simple interest on a principal of $400.00 over 0.25 years (90 days) is 0.05% per day.
