PS

The best way to approach this problem is to pick a value for x first.

For simplicity choose 50 percent.

Thus, if she traveled 50 percent of trip at 40 mph and 50 percent at 60 mph, her average would be 50 mph.

Now fill in 50 for x and see where you get an answer of 5.

C.

Just saw that you have E as answer, are you sure?

Let the total distance be 100

So Distance traveled with 40mph = x

Distance traveled with 60mph = 100-x

Now time to travel x (let t1) = x/40
(because time = Distance/speed)

And time to travel 100 - x (let t2) = (100-x)/60

total time = t1 + t2
Total Distance = 100

Now average speed = total distance/ total time

Avg. Speed = 100 / (t1+t2)

= 100/ [x/40 + (100-x)/60]

take LCM and solve:

= 12000/(x+200)

Answer E

Hope this is clear

Karan

ahh I see how I misinterpretted the question. Thanks for clearing it up.

You are welcome