What's your favorite proof accessible to a general audience? What math statement with proof do you  find most beautiful and   elegant, where such is  accessible to a general audience, meaning you could state, prove, and explain it to a general audience in roughly $5 \pm\epsilon$  minutes. 
Let's define 'general audience' as approximately  an average adult with education and experience comparable to someone holding a bachelor's degree in any non science major (e.g. history) from an average  North American university.

There's no way to tune a piano in perfect harmony.  
There are twelve half-steps in the chromatic scale, twelve notes in each octave of the keyboard.  Start at middle "C", and ascend a perfect fifth to "G".   That's seven half steps up, with a frequency ratio of 3/2.  Drop an octave to the lower "g" -- that's twelve half steps down, and a frequency ratio of 1/2.  Continuing around the "circle of fifths" twelve times, and dropping an octave seven times, brings you back to middle "C", a frequency ratio of 1.  
So, $1 = (\frac{3}{2})^{12} \times (\frac{1}{2})^7$, or $3^{12}=2^{19}$.
Ask your piano tuner next time about those fifths.
