Question 606313
Let the average speed of Jim be 'x' mph
so the average speed of Ann will be 'x+4' mph

let time taken by Jim to travel is 't' hrs.
then the time taken by Ann will be 't-0.25' hrs. (Since 15 mins = .25 hrs)

Now, average speed = total distance travelled / total time taken.

  so,   x = 240/t    --- eq.1

        x+4 = 240/(t-0.25)  -----eq.2


Substituting eq.1 in eq.2


we get           (240/t) + 4 = 240/(t-0.25)  

           =>     1/t + 1/60 = 1/(t-0.25)
           =>     1/(t-0.25) - 1/t = 1/60
           =>      4t^2-t-60 = 0

    So t= 4 or t= -15/4

since time cannot be a negative value.

   t = 4 hrs.
from eq.1   x = 240/t = 240/4 = 60 mph

  So, Ann's speed = x+4 = 60+4 = 64 mph

Ans: 64 mph