You have a $20 bill, and you need to double it. You have 1 month, you must keep it in cash (No banks, cashapp, credit cards), you don’t have an address, a phone number, or and you can’t interact to people in public. using just the internet and an email, how do you double the money?
Sell 40 $1 raffle tickets with a $20 prize, I guess.
That will leave you with $20