Question

A boat is traveling with a velocity of 18 meters/second relative to water and the river is flowing at a velocity of 2.5 meters/second relative to the shore. What will be the velocity of the boat if the boat and the water are in opposite directions?

Answer 1

Vt = Vboat - Vriver 
Vt = 18 - 2.5 = 15.5 m/s 

If the boat's direction is the same as the water, you sum the velocities of the river and the boat . 

Answer 2

Answer:Velocity of boat is 15.5 $\frac{m}{s}$

Explanation:Using equation of relative motion

$V_{boat,water}=V_{boat}-V{water}$

=>$V_{boat}=V_{boat,water}+V_{water}$

where velocity of boat relative to water , $V_{boat,water}$=18 m/s

velocity of water, $V_{water}=-2.5\frac{m}{s}$

and velocity of boat , $V_{boat}=? \frac{m}{s}$ to be calculated.

Putting the known values in the above equation , we get

$V_{boat}= (18-2.5)\frac{m}{s}=15.5\frac{m}{s}$