 Now, ∆L/∆Q is the reciprocal of the marginal product of labor (∆Q/∆L).Therefore, marginal cost is simply the wage rate w divided by the marginal product of labor

Thus if the marginal product of labor is rising then marginal costs will be falling and if the marginal product of labor is falling marginal costs will be rising (assuming a constant wage rate).The average product of labor is the total product of labor divided by the number of units of labor employed, or "Q/L".