求解一道论坛上搜不到的prep数学题。。。

sosols1989

DS: during a 40-mile trip,Marla traveled at an average speed of x miles per hour for the first y miles of the trip and at an average speed of 1.25x miles per hour for the last 40-y miles of the trip. The time that Marla took to travel the 40 miles was that percent of the time it would have taken her if she had traveled at an average speed of x miles per hour for the entire trip? 1) x=48
2) y=20
ans: B
这题实在看不懂。。。小女子无才。。。请求大家帮助。。。感激不尽

doublestar615

是这样的:

问题是问,如果“X和1.25X两种速度分别走Y和40-Y两个距离”，那么这种方法所用的时间是等于“仅用X速度走完整个40距离”的时间的百分之多少。我们分别把两个引号的方法理解为情况1和情况2

所以情况1的时间是= (40+0.25Y) \ (1.25X)

情况2的时间是= (40) \ (X)

接下来就是情况1的时间除以情况2的时间：= (40+0.25Y)(X) \ (40*1.25X)

因为X可以约去，所以我们只要知道Y的值就可以算出最后的百分比。

sosols1989

哇。。果然挺绕的。。。但是解释太清晰了。。十分感谢doublestar615的帮助！！