Solve each problem .
(Modeling) Bacterial Growth The strain of bacteria described in Exercise 85 in the first section of this chapter will double in size and then divide every 40 minutes. Let $a_{1}$ be the initial number of bacteria cells, $a_{2}$ the number after 40 minutes, and $a_{n}$ the number after $40(n-1)$ minutes. (Source: Hoppensteadt, F. and C. Peskin, Mathematics in Medicine and the Life Sciences, Springer-Verlag. )
(a) Write a formula for the $n$ th term $a_{n}$ of the geometric sequence $a_{1}, a_{2}, a_{3}, \ldots, a_{n}, \ldots$
(b) Determine the first value for $n$ where $a_{n}>1,000,000$ if $a_{1}=100$
(c) How long does it take for the number of bacteria to exceed 1 million?