Patrick invests a sum of money for 2.5 years at 12% p.a. compounded monthly. If he earns $11305 after 2.5 years, how much is his investment?

Number of periods, n = 2.5*12 = 30

interest rate per period = 12%/12=1%
I=investment
11305=I*(1+0.01)^30
Solve for I
I=11305/(1.01)^30=8387.44