The amount A on an account after t years, when a principal P is invested with an interest rate r compounded monthly, is given by the formula:
[tex]A=P(1+\frac{r}{12})^{12\times t}[/tex]
The principal that Theo invested was $6600, with an interest rate of 4.5%. Replace P=6600, r=4.5/100 and t=4 to find the value of his investment after 4 years:
[tex]A=6600\times(1+\frac{0.045}{12})^{12\times4}=7898.97\ldots[/tex]
Therefore, the value of his investment after 4 years is $7898.97
