The number of cell phone subscribers in a certain country in the years 2000–2005 was projected to follow the equation N(t) = 39t + 74
million subscribers in year t (t = 0 represents January 2000). The average annual revenue per cell phone user was $350 in 2000. If we assume that due to competition the revenue per cell phone user decreases continuously at an annual rate of 20%, we can model the annual revenue as
R(t) = 350(39t + 74)e−0.2t million dollars.