During the first 4 hours of a trip, you average 40MPH. How long must you travel at 60MPH to finish with an overall average speed of 55MPH?

• avg speed = total distance traveled / total time taken
  55 = (4*40 + x*60)/(4+x)
  220+55x= 160 + 60x
  5x= 60
  x = 6 hrs
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