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
$\displaystyle {f{{\left({t}\right)}}}={\left\lbrace\begin{array}{ccc} {\frac{{{2}}}{{{18}}}}-{\frac{{{2}}}{{{324}}}}{t}&\text{if}&{0}\leq{t}\leq{18}\\{0}&&\text{otherwise}\end{array}\right.}$
Find the median number of minutes that a cheeseburger sits under the heater.

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