Two decision time frames production is fixed. The production and labour is the variable factor of production. The long-run is a period in which the firm can change its plant. A sunk cost is irrelevant to the firm"s current decisions. The increase in total product that results from a one-unit increase in. Diminishing marginal returns: worker exceeds the marginal product of the previous worker additional worker is less than the marginal product of the previous worker. The law of diminishing returns: production with a given quantity of the fixed factors of production, the marginal product of variable factor eventually diminishes. Total cost = total variable cost + total fixed cost. Calculate it by: the increase in total cost divided by the increase in output. Average total cost = average fixed cost + average variable cost. The increase in total cost that results from a one-unit increase in. Average and marginal product and cost curves. A firm"s reaches its lowest level.