a) Can he reach solid land by throwing the rock?
b) If so, how long will it take him?
a.) The man throws rock of mass 5 Kg with speed 12 m/sec, by law of conservation of momentum, the man will get some initial velocity.
m1*v1 = m2*v2
=>60*v1 = 5*12
=> v1 = 1 m/sec
Now, let the man stops after travelling a distance s.
work done ...
with full calculations and explanations, the problem is solved.