The document discusses several economic principles: 1) Opportunity cost refers to the real cost of something being what must be given up to obtain it, not just financial costs. For example, the opportunity cost of a college course is the other classes that could have been taken. 2) The equi-marginal principle states that economic activity should be conducted where marginal costs equal or are lower than marginal benefits. Marginal means incremental. 3) The principle of diminishing returns refers to increasing one input while keeping others fixed, which increases output at a decreasing rate. There is an optimal number of workers where returns are highest.