They're asking because we're all having a little fun at your expense . . .
If there were an easy way to invest $50 when you have no established business, and then grow it to $800 within a few weeks, none of us would be here any more to answer your questions.
We would have invested our $50, gotten $800, reinvested 16 times, gotten $12,800, reinvested 256 more times, and pulled out our $204,000 before someone caught on and arrested us.
The only way I can come up with for you to actually make $800 would be to do the following:
1) Write an incredibly good ebook teaching people how to make an $800 profit in a few weeks with only a $50 investment.
2) Post it for sale for $50.
3) Find 16 suckers that will buy it from you.
Good luck!