What would be the monthly payment for $20,000 at 6.9% financing for 60 months?

that is interesting.  I got the same answer but on the surface our forumulas look different.

$$\begin{array}{rll} A&=&M \left(\dfrac{1-(1+i)^{-n}}{i}\right)\\\\ A\times \dfrac {i}{1-(1+i)^{-n}}&=&M\\\\ M&=&A\times \dfrac {i}{1-(1+i)^{-n}}\\\\ M&=&20000\times \dfrac {0.00575}{1-(1.00575)^{-60}}\\\\ M&\approx &\$ $395.08 \end{array}$$

$395.08

Answer found using a calculator at http://www.mortgagecalculator.org/

Let's see how jboy314's answer was derived.

The "formula" for the monthly payment is given by:

M = P(1+r)ⁿ r / [(1+r)ⁿ-1]

Where

M = the Monthly Payment

P  = the amount financed ($20000)

r = the montly interest rate expressed as a decimal = .069/12 = .00575

So we have

M = (20000*(1+.00575)^60*.00575)/ ((1+.00575)^60 -1)) = $395.08

There you go - I challange someone to do the mathas and show why the formulas are indeed the same. 

Just take the right-hand side of Chris's formula and divide top and bottom by (1+r)ⁿ.  

His equation becomes  M = Pr/(1-1/(1+r)ⁿ) or M = Pr/(1-(1+r)^-n)

Replace his P by your A and his r by your i.  

Thanks Alan.