請教PP上遇到的問題

[lovemifi]

During a trip on an expressway, Don drove a total of X miles. His average speed on a certain
5-miles 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 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?
(A)8.5 ％
(B)50 ％
(C)X/12 ％
(D)60/X ％
(E)500/X ％ =>正確答案
這題我怎麼化簡式子就是化不出正確答案的那個式子 >"<
煩請數學達人解救一下吧~ 感激不進,謝謝

[chour]

T時間=d距離/sp速度
第一段時間=5/30
第二段時間=(x-5)/60
本段旅程總耗時T1= 5/30 + (x-5)/60 =(x+5)/60

如果旅程均速60,則耗時T2=x/60

求(T1-T2)/T2=[(x+5)/60 - x/60 ]/x/60
=5/x=500/x %

[lovemifi]

T時間=d距離/sp速度
第一段時間=5/30
第二段時間=(x-5)/60
本段旅程總耗時T1= 5/30 + (x-5)/60 =(x+5)/60

如果旅程均速60,則耗時T2=x/60

求(T1-T2)/T2=[(x+5)/60 - x/60 ]/x/60
=5/x=500/x %

蛤~原來是這樣簡單啊
chour真是謝謝你