反证法:Assume no 4 or more consecutive occupied chairs. Then there are at most 3 consecutive occupied chairs, wich each group of consecutive occupied chairs seperated by at least 1 unoccupied chair. So, 61 people can be divided into at least [61/3]+1=21 groups with 21-1=20 unoccupied chairs between each groups. But 61+20>80. So, there must be some group with 4 consecutive occupied chairs.