Answer
The walker takes 0.75 hour and the rower takes 1.47 hours.
Work Step by Step
The walker travels 3.0 km at a steady speed of 4.0 km/h. The time is $\frac{3km}{4km/hr}= \frac{3}{4} hour$.
We calculate the rower’s speed relative to the shore for each segment of the round trip. With the current, the boat moves at 6.80 km/h, and against the current, the boat moves at 1.2 km/h relative to the shore.
The rower takes a time of $\frac{1.5km}{6.80km/hr}+\frac{1.5km}{1.2km/hr}=1.47h$
