The concentration of a drug in an organ at any time $t($in seconds$)$ is given by

$C(t)=\{\begin{array}{ccc}0\text{:}3t-15(1-e^{-\frac{t}{60}})& if& 0t20\\ 15e^{-\frac{t}{60}}-9e^{-\frac{t-20}{60}}& if& t>20\end{array}$

where $C(t)$ is measured in milligrams$/$cubic centimeter $(m\frac{g}{c}{m}^3).$ What is the concentration to four decimal places of the drug in the organ after $20sec?$