Step-by-step explanation:
formally, for the 60 mph average, she needs to go these 2×20=40 miles at 60 mph. that means she would need 40/60 = 2/3 hours = 2×60/3 = 40 minutes.
going the first direction at 50 mph these 20 miles took her 20/50 = 2/5 hours = 2×60/5 = 24 minutes.
that leaves only 16 minutes for the drive back.
16/60 = 4/15 hours
now, she needs to go x mph to drive the 20 miles in 16 minutes (4/15 hours) :
20/x = 4/15
20×15 = 4x
x = 5×15 = 75 mph
