Question

A plane left Chicago at 8 A.M. At 12 P.M., the plane landed in Los Angeles, which is 1,700 miles away.

Answer 1

Commercial passenger flights always list the time AT THE PLACE where
the event took place.  The data you gave in your question means that the
flight departed ORD at 8:00 AM Central (Chicago) Time, and arrived LAX
at 12:00 PM Pacific (Los Angeles) time.

Since the trip spanned two time zones, it was actually in the air for 6 hours.

Average speed = (distance) / (time to cover the distance)

                         = (1,700 miles) / (6 hours)  =  (283 and 1/3) miles per hour.

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But that wasn't what you had in mind, was it.

You meant that the flight took 4 hours.

In that case, the average speed was

                   (1,700 miles) / (4 hours)  =  425 miles per hour.

This is a much more reasonable average speed for a long haul
passenger jet airliner.