A cannon of mass 800\,kg rests on a horizontal rough surface (coefficient of kinetic friction μk=0.20). It fires a 20\,kg shell horizontally at 400\,m\,s−1 relative to the ground. Take g=10ms−2 and neglect the mass of gases. How far will the cannon slide back before coming to rest?
a) 24\,\text{m}
b) 25\,\text{m}
c) 12.5\,\text{m}
d) 50\,\text{m}