You put $400 in a account the amount earns $18 simple interest in nine months what is the annual interest rate

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.