Question 401155
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Hi
Let x represent time traveled by each of the planes
Question states***
 750kmph*x + 850kmph*x = 4000km
       x = 4000km/1600kmph
       x = 2.5hr, time traveled when 4000km apart

CHECKING our Answer***
  750*2.5 + 850*2.5 = 4000km