Exponential Functions

In 2.2, Nova's subscriber count became the function $N(t) = 256(1.5)^{t}$. Now we step back and study the whole family that $N$ belongs to. Not every exponential function grows: a cup of coffee is a good example of one that shrinks. After you drink 160 mg of caffeine, your body removes about 13% of whatever is left every hour, so the amount left after $t$ hours is