The economic efficiency of the third unit of input is .75. The economic efficiency of the second unit of input is 2.5. The optimal quantity of inputs to employ is therefore 2 units. The rule to follow is to employ an additional unit of input if its marginal benefits are greater than or equal to its marginal costs. In this example, marginal benefits are called change in total returns and marginal costs are called change in total salary.
Economic efficiency is actually marginal benefits divided by marginal costs.