A crowd watching air balloons at night .
It is probably the case that there are 20 balloons in the air and 50 on the ground .
It is impossible that there are 20 balloons in the air and 50 on the ground .
It is probably the case that there are 20 balloons in the air and 50 on the ground .
There are 20 balloons in the air and 50 on the ground .