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Julie drives 100 mi to Grandmother's house. On the way to Grandmother's, Julie drives half the distance at 33.0 and half the distance at 80.0 . On her return trip, she drives half the time at 33.0 and half the time at 80.0 . What is Julie's average speed on the way to Grandmother's house? What is her average speed on the return trip?

Answer :

Average speed is ALWAYS calculated as total distance / total time 
total distance is 100mi 
total time is (50mi / 33) + (50mi / 80) 
total time is = 1.52 + 0.625 = 2.145 
so average speed going to grandmothers house is 100mi / 2.145 hrs = 46.62 mph 

on her return trip 
x = V1 x t/2 + V2x t/2 
100 = 33t/2 + 80t/2 
100 = 113t/2 
t = 100x2/113 = 1.7699hrs 
average speed = 100mi/1.7699hrs = 56.60mph

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