The Bode magnitude plot of the transfer function $G(s)=\dfrac{K(1+0.5s)(1+as)}{s(1+\frac{s}{8})(1+bs)(1+\frac{s}{36})}$
is shown below:

Note that $-6dB/octave = -20 dB/decade.$ The value of $\dfrac{a}{bK}$ is ________.