gwd7-37，求助

yangxiaogang

Q37:

During a trip, Francine traveled x percent of the total distance at an average speed of 40 miles per hour and the rest of the distance at an average speed of 60 miles per hour.  In terms of x, what was Francine’s average speed for the entire trip?

A.    (180-x)/2

B.     (x+60)/4

C.     （300-x）/5

D.     600/（115-x)

                     E.     12000/(x+200)

怎么做啊？觉得答案有问题

yangxiaogang

UP

nyc_cfa

S= total distance; T = total time

T= S*(x/100)/40 + S*(1-x/100)/60

Average Speed = S/T = S/[S*(x/100)/40 + S*(1-x/100)/60] = 1/[x/4000+(1-x/100)/60] = 1/[x/4000+(100-x)/6000] = 12000/[3x+200-2x] = 12000/[x+200] = E

jingle0915

think