A kayaker can row at a steady rate of 12 mph in still water. A river has a current of 4 mph. How long does it take round-trip if she rows 36 miles upstream and then rows back to where she started?

Net speed upstream = 12 - 4 = 8 mph
Time to paddle 36 miles upstream = 36/8 = 4.5 hours
Net speed downstream = 12+4 = 16 mph
Time to paddle downstream = 36/16 = 2.25 hours
Time for round trip = 4.5 + 2.25 = 6.75 hours = 6 h 45 m

Simply paddling 2x36 miles in still water would only take 6 hours.
The kayaker spends more time fighting against the current than being pushed by it so the round time is longer the 6 hours