Is not a bitcoin it's 100 in USD he can only get this 0.0015166451808599BTC if he buys bitcoin at this rate $65,935.
And how did you actually get 31 dollars? He gains are only $1.5 when he sold this 0.0015166451808599BTC at $66,935
Yes, I mean $ 100, if you read it you will know that I performed calculation correctly. The error was rhetorical.
If he invests $ 100, average price at $ 51,000, and sells at $66,935 he will make $ 31 profit. try it and will find he will makes 31 dollar if stats now and price at $66,935