Two planes leave O'Hare Airport in Chicago at the same time. One flies directly east at 540 mph. The other flies directly west at 720 mph. How long will it take them to be 1890 miles apart?

• in 3.5 hours
  
  let us suppose that in 't' hours they will be 1890 miles apart.
  Therefore, distance travelled by plane first in 't' time = 540*t= 540t
  similiarly , distance travelled by plane second in 't' time = 720*t = 720t
  
  so, 1890 = 540t + 720t
  t=7/2 = 3.5 hours
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