$6 \cdot 6$ Calculate $4\left(\cos 15^{\circ}-i \sin 15^{\circ}\right) \cdot 5\left(\sin 15^{\circ}-i \cos 15^{\circ}\right)$, and express the product in the form $a+$ $b i$.
\begin{array}{l}\text { [Solution] Original expression }=20\left(\cos 15^{\circ}-i \sin 15^{\circ}\right)(-i)\left(\cos 15^{\circ}+i \sin 15^{\circ}\right) \\ =(-20 i)\left(\cos 0^{\circ}+i \sin 0^{\circ}\right) \\ =-20 i \text {. } \\\end{array}