Question

A boat is rowed across the river at rate of 9 km per hour the river flows at a rate of 12 km per hour the velocity boat is

Answer 1

velocity of boat with respect to river water flow is 9km/hr.(given)
velocity of river flow is 12km/hr.
so as we know that
V of boat wrt water=V of boat -V water
9 =Vboat -12
Vboat =21 km/hr....

I hope it helps u

Answer 2

The velocity of river flow is 12 km/hr
And, The velocity of the boat with which it is rowed (across the river means it will be perpendicular) is 9 km/hr

Therefore, the net velocity of boat will be the sum of both velocities.

$12i + 9j \\ \sqrt{ {12}^{2} + {9}^{2} } \\ \sqrt{144 + 81} \\ \sqrt{225} \\ 15$
Hence, the answer would be 15 km/hr.