The Bode magnitude plot of the transfer function $G(s)=\frac{K(1+0.5s)(1+as)}{s(1+\frac{s}{8})(1+bs)(1+\frac{s}{36})}$
Note that $-6$ $dB$/$octave$ = $‒20$ $dB$/$decade$. The value of $\frac{a}{bK}$ is ______.