it doesn't, it's a trick so the person who sold the box ends up with more money than the person who bought it
for easy explaining
person a = person selling box
person b = person buying box
so person a and person b both initially lose 20 dollars to the box
person b then gives person a 30 dollars
now person a has + 10 dollars (he lost 20 to the box, and gained 30 from selling it)
HOWEVER
person b has -50 dollars (he lost 20 to the box, and lost 30 from buying the box)
now person b gains 40 dollars from buying the box, and is left with a total of -10 dollars
/thread