Suppose that the sales at Borders bookstores went from $64$ million dollars in $1991$ to $405$ million dollars in $1996$. Find an exponential function to model the sales (in millions of dollars) as a function of years, $t$, since $1991$.
sales = million dollars

What is the continuous percent growth rate, per year, of sales?
continuous growth rate = percent
(Enter your answer as a percent, not a fraction.)