When going to work, her speed is r = 60 mph. She takes t = d/r = 80/60 = 4/3 hours which converts to 80 minutes. Multiply by 60 to go from hours to minutes.
Notice how the '80' shows up twice (in "80 miles" and "80 minutes"). This is because traveling 60 mph is the same as traveling 1 mile per minute.
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Now as she's coming home, her speed becomes r = 40 and she takes t = d/r = 80/40 = 2 hours = 120 minutes.
The difference in time values is 120 - 80 = 40 minutes.
Her commute back home takes 40 more minutes compared to the morning drive to work.
