# Math Help - Compound Interest

1. ## Compound Interest

I am having trouble coming up with the number for n :

Lisa will invest some money on July 3, her sixteenth birthday, at 4.5% per year, compunded monthly. How much should she invest if she wants to have \$ 10,000 on the November 3 following her eighteenth birthday?

I have found

P= 10,000
i= 0.045/12= 0.00375
n= ? ( not sure how to get this)

Thank you for the help

2. This formula should do the trick..

$P = \frac{A}{\left(1 + \frac{r}{n}\right)^{nt}}$

where P is the amount she should invest, A=10000, r is the rate, n is the number of times the interest is compounded per year, and t is number of years, here 24+4/12..