Saving $100 per month at 12% interest over 20 years. How would I write the formula?

Well it depends when the money goes into the bank

Say it goes in at the end of each month. And the interest is really paid monthly.

Then you would be getting 1% interest per month for 20*12 = 240 months

This is the future value of an ordinary annuity problem.

$${\mathtt{100}}{\mathtt{\,\times\,}}\left({\frac{\left({\left({\mathtt{1}}{\mathtt{\,\small\textbf+\,}}{\mathtt{0.01}}\right)}^{{\mathtt{240}}}{\mathtt{\,-\,}}{\mathtt{1}}\right)}{{\mathtt{0.01}}}}\right) = {\mathtt{98\,925.536\: \!538\: \!736\: \!004\: \!474\: \!5}}$$

So you would have    $98,925.54      

((100*12)*12/100)*20. Hope this helps :)