Read it properly, they said they would sell the box, never said the money would be part of the sale. If the box costed $10, then it leaves them with $20, split between the two, is $10 profit each, whilst taking out the $40 they had put in, the buyer only buys the box for $30
Unless they selling it to each other, then they would be making nothing as it would just go back and fourth
Ok let's name one guy John and the other guy Harry. Both of them put in $20, which means they lose $20 each. Currently both John and Harry have $-20. John buys the box for $30, which mean he has a total of $-50, while Harry gains $30, he has a total of $10. In the box that John receive is $40, so he has a total of $-10. In the end Harry has a profit of $10 while John loses $10. The box does not really matter
Oh I get it now after reading 100 times haha. It's saying 'Harry' makes ten as he paid $30 for the box and got $40 = $10 profit And 'John' makes ten as he only put $20 in the box, and gets $30 for it = $10profit It kinda forgets the initial money that Harry put in, in the first place lol. But yeah, I get what it's trying to joke about finally.