Thank you for the opportunity to help you with your question!
The concept that is used to solve this question is Law of conservation of momentum. It says that if two inelastic object of masses m1 and m2 and velocities v1 and v2 respectively collide than always the sum of their net momentum is zero. i.e. m1v1+m2v2 = 0
f) Force is defined as rate of change of momentum. Now let us take the momentum of bullet which is m2v2 = 0.025*800 = 20Kgm/s
Hence force = 20/0.05 = 400N
g) Let the velocity of rifle be v1, then by law of conservation of momentum
m1v1+m2v2 = 0
So 12*v1 = 0.025*800
v1 = 20/12
Please let me know if you need any clarification. I'm always happy to answer your questions.