此题怎么也算不到答案，是否我理解有误？

redli

是否我理解
In a given river, the current is 6 mph. A certain riverboat can travel 18 mph in still water. How far upstream (against the current) can the boat travel if a round trip is to take 10 hours?

A) 24

B) 48

C) 60

D) 80

E) 96
答案选D

Louisa_xy

逆流能走多远-求单程距离？
船先逆行，速度=18-6=12mph。  后顺着水流，速度=18+6=24mph  
往返走的距离一样但所用时间不同，设单程距离x
x/12 + x/24 =10    x=80