提问几道GRE数学题!

题目

提问几道GRE数学题!
都是绿皮上的,本人数学不大好,
P275.39
In how many of the integers between 1 and 100 does the digit 5 occur exactly once?
答案是18,题目啥意思,如何做?
P231.2
A train travels 60 miles per hour for 3 hours and then 45 miles per hour for 2 hours.What is the train's average speed in miles per hour during the 5-hour period?
P246.17
A teacher has a total of at least 110 students enrolled in her classes.If the teachers has an average of exactly 27 students enrolled per class,what is the least possible number of classes that the teacher could have?
我算得4呀

答案

题目是问在1和100之间的整数中,有多少个5只出现1次.5,15,25,35,45,65,76,85,9550,51,52,53,54,56,67,58,59,共18个(55不算)The total distance is 60*3 + 45*2 = 270 miles,total time is 5 hours,evarage speed is 2...