Answer:

Explanation:
We have been given that your friend borrows $100 from you and promises to pay you back $109 in 8 months.
We will use simple interest formula to solve our given problem.
, where,
A = Amount after t years,
P = Principal amount,
r = Annual interest rate in decimal form,
t = Time in years.
Convert 8 months to year:










Convert to percent:

Therefore, you are charging 12% APR to you friend.