The revenue per month earned by the Couture clothing chain at time t is

R(t) =N(t)S(t),

where N(t) is the number of stores and S(t) is the average revenue per store per month. Couture releases a statement to their investors saying that currently they have 50 stores with a total revenueof$7,500,000 and expect to increase the number of stores by about 2 stores per month and increase total revenue by about$250,000 per month. What does this statement imply about the current rate of change of the average revenue per store per month?