請教GWD18-14

[yencm]

題目:
A certain pilot flew 400 miles to City K at an average speed of 350 miles per hour with the wind and made the trip back at an average speed of 250 miles per hour against the wind.Which of the following is closest to the pilot's average speed,in miles per hour,for the round trip?
A.280
B.290
C.300
D.310
E.320

我知道平均速率是總距離除以總時間，我的想法是去程的時間為400/250，而250是本身速度350-風速100，所以回程的速度我也把它想成是350+風速100=450(就像以前國中那種順流逆流船速的想法)，因此解出來的答案是E.320

但看解答似乎回程速度是以350計算，請問為何不用加上順風速呢?