I did systems of many particles and variable mass systems. I learned finding center of mass with integrals and interactions with variable mass so like snow falling on train or rockets, and derived the tsiolkovsky rocket equation. The variable mass systems reminded me of this old problem of snow falling on a train. I remember being stumped and struggling to work it out, and one day at school when I was bored, I finally solved it on a table with a white board marker. Although the solution I found was not too complicated, it is a bit challenging to think of. The calculus approach, instead, is actually more intuitive and allows you to solve it mentally.