题目

Birds that are half-asleep—with one brain hemisphere (半球) alert and the other sleeping—control which side of the brain remains awake, according to a new study of sleeping ducks.     Earlier studies have documented half-brain sleep in a wide range of birds. The brain hemispheres take turns sinking into the sleep stage characterized by slow brain waves. The eye controlled by the sleeping hemisphere keeps shut, while the wakeful hemisphere’s eye stays open and alert. Birds also can sleep with both hemispheres resting at once.     Decades of studies of bird groups led researchers t（本题满分16分）某市居民自来水收费标准如下：每户每月用水量不超过4吨时，按每吨1.8元收费；当每户每月用水量超过4吨时，其中4吨按每吨为1.8元收费，超过4吨的部分按每吨3.00元收费。设每户每月用水量为吨，应交水费元。 （Ⅰ）求关于的函数关系； （Ⅱ）某用户1月份用水量为5吨，则1月份应交水费多少元？ （Ⅲ）若甲、乙两用户1月用水量之比为，共交水费26.4元，分别求出甲、乙两用户该月的用水量和水费。