The optimal combination of resources is achieved through cost-minimization and profit-maximization.

The Least-Cost Rule

A firm is producing a specific output with the least-cost combination of resources when the last dollar spent on each resource yields the same marginal product.

Marginal product Marginal product
of labor (MPL) = of capital (MPc)
Price of Labor (PL ) Price of Capital (Pc )

The Profit-Maximizing Rule

In competitive markets, a firm will achieve its profit-maximizing combination of resources when each resource is employed to the point at which its marginal revenue product equals its resource price.
MRPL/PL = MRPC/PC = 1 (*ratios must equal 1)