Using the link below please solve question number 1 one step

Using the link below please solve question number 1 one, step by step and neatly so i may fully understand, I thank you in advance!

http://imgur.com/iXCdgdO

Assuming all the weights are in lb <----- as it is not mentioned in the question. I have solved the question accordingly.

Now, initital momentum of the system is 0 <----- as the system is not in motion

After the rock has been throuwn with velocity 25 mph.

the momentum of the rock = 20*25 = 500 lb.mi/h

momentum of the man+boat = (170+30)*v = 200*v

v = velocity of the boat after throwing

So, by conervation of momentum, total momentum must not change

So, 200*v + 500 = 0

So, v = -2.5 mph <--------- direction is opposite to the direction of throw