Zero Dash Four Dash Twenty

Everyone is born with a number on their wrist; the higher the number, the better off you are in society. A Sum is another person that has a number that, when added to another's, equals twenty; a strong emotional connection is often felt between Sums, but Sums are not required to be romantically or sexually involved.
Zeros are rare. So are Twenties. But fate always has the upper hand, doesn't it?