At a particular fast-food restaurant, food is often made in advance, and then it is placed under a heater until it is sold. The distribution function for the number of minutes that passes between when a cheeseburger is made and when it is sold is given by $$f(t) = \begin{cases} \frac{2}{9} - \frac{2}{81}t & \text{if} \;\; 0\leq t \leq 9\\ 0 & \text{otherwise.} \end{cases}$$ Find the median number of minutes that a cheeseburger sits under the heater.