Answer: Sound waves are a waves of compression and rarefaction, by which sound is propagated in an elastic medium such as air.
Explanation:
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
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 .
As we know that as per Newton's II law we have

here we will have
= change in momentum
= time interval in which momentum is changed
now in order to have least injury during jumping we need to have least force on the jumper
so in order to have least force we can say that the momentum must have to change in maximum time so that amount of force must be least
So we need to increase the time in which momentum of the system is changed