-- When the 7.1 and 12.6 sides both point in exactly the same direction,
their open ends are 5.5 apart. So the missing side must be greater than 5.5 .
All of the choices are greater than 5.5 , so we haven't found it yet.
-- When the 7.1 and 12.6 point in opposite directions, their open ends are
19.7 apart. So the missing side can't be greater than 19.7, otherwise the
two sides we know couldn't reach between its ends. Choice 'E' is greater
than 19.9, so it can't be the missing side.
