某校校长暑假将带领该校市级“三好学生”去北京旅游，甲旅行社说：“如果校长买全票一张，则其余学生可享受半价优惠．”乙旅行社说：“包括校长在内全部按全票价的6折优惠”（即按全价的60%收费），若全票价为240元。
（1）设学生数为x，甲旅行社收费为y甲，乙旅行社收费为y乙，分别计算两家旅行社的收费（建立表达式）；
（2）当学生数是多少时，两家旅行社的收费一样？
（3）就学生数x讨论哪家旅行社更优惠。
解：（l） y甲=120x+240，
y乙=144x+144；
（2）由y甲= y乙，则120x+240=144x+144，
解得x=4；
（3）若y甲>y乙，即120x+240>144x+144， 解得x<4，
若y甲<y乙，即120x+240<144x+144，解得x>4，
所以当学生数少于4人时，乙旅行社更优惠；
当学生数大于4人时，甲旅行社更优惠。
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