Let p represent the speed of the plane in miles per hour.
time = distance/speed
.. 420/p = 180/(p -80)
.. 420(p -80) = 180p . . . . . . "cross multiply"
.. (420 -180)p = 420*80 . . . add 420*80 -180p
.. p = 33600/240 = 140
The speed of the plane is 140 mph.
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Check:
.. Each trip takes 3 hours = 420/140 = 180/60
