The marginal product of one additional worker is known as the "marginal product of labor" (MPL). This is the additional amount of output that will be generated when another worker is added. It is marginal in the sense that it varies with the quantity. Therefore, we are looking for a rate of change.
MPL = change in L/change in quantity
As it describes production, it is found in the production function and will vary according to the other variables in the function. In this production function:
Y = 25KL
Y = output (product), K = capital (number of telephones), and L = labor (number of workers)
Intuitively, it makes sense that the amount of output depends on the levels of capital (telephones the workers have to work with) and the number of workers.
Since MPL is a rate of change, it is found by deriving the function with respect to L. So taking the partial derivative yields:
dL/dY = 25K = MPL
This is a linear relationship, as the marginal product of any additional worker at any production level will always be the amount of telephones K multiplied by 25. Thus, to state the answer in terms of K:
MPL = 25K