After a special medicine is introduced into a petri dish containing a bacterial culture, the number of bacteria remaining in the dish decreases rapidly. The number of bacteria decays by a factor of $\dfrac{1}{15}$ every $6.7$ minutes, and can be modeled by a function, $N$, which depends on the amount of time, $t$ (in minutes). Before the medicine was introduced, there were $90{,}000$ bacteria in the Petri dish. **Write a function that models the number of bacteria $t$ minutes since the medicine was introduced.** $N(t) = $ [[☃ expression 1]]