Answer
$\approx67,121.04$
Work Step by Step
In $A(t)=P(1+\frac{r}{n})^{nt}$ for compound interest, $P,r,n,t$ respectively stand for the principal, interest rate per year, the number of times the interest is compounded per year and the number of years. $A(t)=100000$ is the amount after $t$ years.
So if we invest $P$ at an interest rate of $r=0.08$ compounded monthly ($n=12$), the amount after $t=5$ years is:
$100000=P(1+\frac{0.08}{12})^{12(5)}\\P=\frac{100000}{(1+\frac{0.08}{12})^{12(5)}}\approx67121.04$
