I'm writing a story about a 20-year-old who wins the Powerball and starts an aerospace company and it blows up and he is a billionaire by 27. Is this believable?

If you write it convincingly, anything can be believable. It's about the frame you place it in. And it's fiction, it doesn't have to be the most realistic thing ever. Have fun with it. It's about the joy it brings you, not whether or not your fiction novel is realistic enough. Otherwise, we wouldn't have all the great literature we do today.