Final answer:
Without specific cost and production data, we cannot calculate the marginal costs for 3 and 4 laborers. However, a firm in perfect competition maximizes profit by hiring up to the point where the market wage equals the marginal revenue product.
For production methods, the firm should choose the method with the lowest total cost given the new machinery cost.
Step-by-step explanation:
The original question seems fragmented and lacks context, but it appears to be aiming towards calculating the marginal cost of labor for producing goods. Marginal cost is the change in total cost that arises when the quantity produced changes by one unit. To compute the marginal costs, more details such as the initial production costs and the change in output with each additional laborer are necessary. Unfortunately, without these details, it's impossible to calculate the precise marginal cost values for 3 and 4 laborers.
In a theoretical scenario where a firm operates in perfect competition and pays a market wage of $12, the profit-maximizing level of employment would be the point at which the marginal product of labor is equal to the market wage because the cost of hiring an additional worker ($12) is covered by the additional revenue the worker generates.
If the marginal revenue product of each worker is $13, the firm would continue to hire until the marginal revenue product equals the wage rate. Therefore, the firm maximizes profit by employing workers up to the point where the wage rate is equal to the marginal revenue product.
If the monopsonist's marginal cost of labor at each level of employment was provided and the marginal revenue product of each unit of labor is $13, then the firm's profit-maximizing level of employment would be at the point where the marginal cost of labor equals the marginal revenue product.
Returning to the production method choice with updated costs: If the cost of labor remains $40 but the cost of a machine decreases to $50, the total cost for each production method needs to be recalculated considering the new cost of machinery. The firm should select the method with the lowest total cost that still meets production needs.