Chapter 12.2, Problem 54E

Cellphone Revenues (Refer to Exercise 53.) If we assume instead that the revenue per cellphone user decreases continuously at an annual rate of 20%, we obtain the revenue model

$R\left(t\right)=350\left(39t+68\right)e^{-0.2t}\text{ million dollars}\text{.}$

Determine (a) when to the nearest 0.1 year the revenue was projected to peak and (b) the revenue, to the nearest $1 million, at that time.