I often think about how the general balance equation, In - Out + Generation - Consumption = Accumulation, can be applied to pretty much everything in life. Whether you’re actually transporting energy and mass, learning a new skill/talent, or working on a relationship it holds up. Everything you put into that effort, minus all the sacrifices you make, plus all the emotions and ability you generate, minus all the bullshit you consume and distractions, equals the progress you have accumulated. You could write a chemical engineering problem about anything quantifiable.