To find the amount of money that you must pay per period to pay back a loan, use this formula:
A = [P·r(1 + r)^n] / [ (1 + r)^n - 1]
where: A = payback amount per period
P = initial loan amount
r = interest rate per period (example 9% per year rate paid monthly: 0.09/12)
n = total number of payments (example: monthly payments for 8 years: 12·8)
If all that you want is an answer, you may try this site: http://www.amortization-calc.com/
Approximately 30.85%
Once you know the standard deviation value, the other values aren't necessary to find the percentage.
If you need a calculator, go to this site: http://www.danielsoper.com/statcalc3/
Choose 'Cumulative Distributive Function (CDF) Calcuator for the Standard Normal Distribution'.
When you are at the correct page, enter 0.5; its response is the percent below the value that you entered.