Question 363355
 Let S = Speed
     T = travel time
     D = distance traveled

Given:
     S1 = 40
     S2 = 60 

Ave S = Total distance traveled divided by total travel time
      = 2D / ( T1 + T2)
      = 2D / (D/40 + D/60)
      = 120 * 2D / [120*(D/40) + 120*(D/60)]
      = 240 D / (3D + 2D)
      = 240/5
      = 48 miles per hour