The following data show the total output for a firm when different amounts of labour are combined with a fixed amount of capital. Assume the wage per unit of labour is $10 and the cost of the capital is $50.

Labour per period Total output per period

0 0

1 10

2 30

3 90

4 132

5 150

TABLE 7-3

Refer to Table 7-3. If this firm is producing 111 units of output per period, its marginal cost is