100000=P(1+(0.08/12)^12*40

100000=P(1+(0.08/12))^(12*40)

So...I'm assuming we're looking for some principal invested at 8% and compounded monthly for 40 years which accumulated to $100000??

So .....

P = 100000/(1+(0.08/12))^(12*40) =

$${\frac{{\mathtt{100\,000}}}{{\left({\mathtt{1}}{\mathtt{\,\small\textbf+\,}}\left({\frac{{\mathtt{0.08}}}{{\mathtt{12}}}}\right)\right)}^{\left({\mathtt{12}}{\mathtt{\,\times\,}}{\mathtt{40}}\right)}}} = {\mathtt{4\,119.738\: \!461\: \!084\: \!481\: \!01}}$$

So....the principal was about $4119.74