求助prep上一道数学题

serendipityln

During a trip on an expressway, Don drove a total of x miles. His average speed on a certain 5-mile section of the expressway was 30 miles per hour, and his average speed for the remainder of the trip was 60 miles per hour. His travel time for the x-mile trip was what percent greater than it would have been if he had traveled at a constant rate of 60 miles per hour for the entire trip?
8.5%   50%   (x/12)%    (60/x)%    (500/x)%

答案是最后一个~突然蒙了，算了好几遍都不对，请哪位同学能给解答一下~~

doreendu0809

一个是 5/30+(x-5)/60=(x+5)/60
还有就是X/60
第一个比第二个多5/60
也就是（5/X)*100%=500/x%

好运9月9

【正常的时间-60m/h的时间】/  60m/h的平均时间  *100%

serendipityln

哦~~明白了~~Thank you~~