How do i calculate marginal product of labor??

Okay here's the problem: Q=5KL, L=Labor, Q=# of gadgets, K = capital in machine hours, labor =$20 per hour, capital= $80per hour (machine rental charge), budget for the month is $64,000. to pay for both labor and capital.... The question is find the optimal capital/labor ratio and how do you maximize the firm's output?????