The answer to the riddle is:
Window Washing. You take the $20 to the nearest supermarket and buy a bottle of DW soap, a pail and a squeegee. You start walking up the street in any business district offering to wash store windows for $5/10, whatever the going rate is which you'd figure out pretty quickly. In the right neighborhood, you could be making $50 hour if you're good.
Very soon, you'd have a nicer bucket and equipment and maybe an assistant. Rinse and repeat until you're a millionaire.
You're welcome.
