Answer:
20
Explanation:
Marginal physical product can be defined as a change in the physical product as a result of an extra unit of labor input , assuming the quantities of other input remain constant.
This is calculated by dividing the change in physical product by the change in the unit of labor.
Workings
    Units of labor       Units of physical products
          0                    0
          1                    50
          2                    110
          3                    155
          4                    185
          5                    205
4 units of labor = 185
5 units of labor = 205
Marginal physical product = 205-185/1
=20