出了270；老板拿250，伙计拿20；
270=250 20
跟290/300没有关系啊？

the way we have to think like this:
1,for staying in this hotel they have to pay 250 any way.
2,the boy take his tip by not letting them know so they have to pay 20 more.
3,250 20=270
4,they have 30 refund so 300-30=270
5,they paid hotel and boy's tip in total of 270, 90 each.

90元 X３人=270元=服务生的20元 老板 250元

３人的270元 - 服务生的20元 = 老板只要的250元

3*90 30 =300

题目把不同公式中的数据混在一起算，就乱了