Your example proves nothing about the boundedness of the rationality of (most) humans. It tells you something about what people expect about other people: other

Your example proves nothing about the boundedness of the rationality of (most) humans. It tells you something about what people expect about other people: others are not expected to carry an argument to its logical conclusion. However, taking that knowledge, or educated guess, into account, makes '10' or '20' a fully rational guess, based on lack of complete data.