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 80 miles per hour. In terms of x, what was Francine's average speed for the entire trip?
Choose the correct choice
A) (180-x)/2  B) 4800/x+60  C) (300-x)/5  D) 600/(115-x)  E) 12,000/(x+200)
total distance =100 ;total time = T   t1 = x/40 t2 = 100-x /80   T =t1+t2 = x/30 + 100-x/80 = (80x + 3000- 30x)/2400   = (50x+3000)/2400   = x +60 / 48   av speed = 100*48/x+60 = 4800/x+60   4800/x+60
<result>B</result>
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 80 miles per hour. In terms of x, what was Francine's average speed for the entire trip?