设每辆甲种货车运货$x$吨，每辆乙种货车运货$y$吨，
得$\left\lbrace \begin{align} & 2x + 3y = 15 . 5 , \\ & 5x + 6y = 35 , \\ \end{align} \right .$解得$\left\lbrace \begin{align} & x = 4 , \\ & y = 2 . 5 . \\ \end{align} \right .$
运费：$ ( 3 \times 4 + 5 \times 2 . 5 ) \times 30 = 735$ （元）
