GWD18-14

[vickyhung]

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?

可以請各位大大幫忙嗎??
算不出答案......

[scujean]

1.請把題目貼齊

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

[解]距離/時間=速率 時間=距離/速率
平均速率=總距離/總時間 →[400*2]/[（400/350）+（400/250）]=約290