Here’s a hint: Don’t use the $5.
This is essentially the first idea Tina Seelig discusses in her book What I Wish I Knew When I Was 20. I highly recommend this book because it talks about a lot of ideas that you don’t learn in school (like the above riddle).
Getting back to the riddle, in the book Tina discusses how she gave this question as an assignment to one of her classes at Standford University. The first responses from students were get “rich quick schemes” such as buying a lottery ticket, or going to Vegas. Then the typical responses ensued (which I must admit, is all I could muster) such as a car wash, or lemonade stand. However, the surprising thing is that some groups were able to make $500 despite (or in spite) of only having $5 in start-up capital.
The hint is pointing you in the direction of “thinking outside the box”. The big idea here is that you should realize that $5 is just as good as nothing, so re-phasing the question as “How do you make $500 in two hours starting from absolutely nothing?” gives us a significant shift in mentality. This allowed all the groups to get very creative beyond what they could have done with the $5. I’ll leave it as an exercise to either think about how to do it, or just buy the book.
But… along those same lines of thinking, here’s another riddle: I have a degree in X, how can I get to Y in five years? Here’s a hint: you don’t necessarily need to use X.