A 0.14-km wide river flows with a uniform speed of 4.0 m/s toward the east. It takes 20 s for a boat to cross the river to a point directly north of its departure point on the south bank. What is the speed of the boat relative to the water?
1) 5.7 m/s
2) 8.5 m/s
3) 8.1 m/s
4) 7.0 m/s
5) 6.4 m/s
Answer: 3