Just take a quick look at this lot:
http://coinmarketcap.com/currencies/views/all/If youve only got $40 to invest,and you just want to gamble, ignore the top currencies and put it into something in the 100-250 range. If one of these coins rises like crazy, it could give you a real profit.
You also have a good chance that you will never see your $40 ever again......do lots of research.
If you're prepared to day-trade and sit in front of your PC all day......put it into one of the top 100 coins, sell out every time you see a 10-20% profit and re-invest.
With some luck and good choices, you could grow your $40 to a reasonable sum, but again...research.
Lastly: if you want to make money in crypto, like almost everything else, it's a lot easier if you can start out with a lot of money.
