In the superposition of two coherent waves (at x=0) given by $y_1=4\sin 300\pi t$ and $y_2=2\sin 308\pi t$. 

$y=y_1+y_2=4\sin 300\pi t+2\sin 308\pi t.$

 If we ignore unequal amplitudes

$y=\sin 300\pi t+\sin 308\pi t$

 $=2\sin \frac{300\pi t+308\pi t}{2}\cos \frac{300\pi t-308\pi t}{2}$

 $=2\cos 4\pi t\sin 304\pi t$

 $=A\cos 4\pi t\sin 304\pi t$.

Frequency of amplitude variation $=\frac{4\pi }{2\pi }=2Hz$.

Resultant frequency $\frac{\omega }{2\pi }=\frac{304\pi }{2\pi }=152Hz$.

Number of beats = $n_2-n_1=4Hz$

$\frac{l_{\max }}{l_{\min }}=\frac{{(a_1+a_2)}^2}{{(a_1-a_2)}^2}=\frac{{(4+2)}^2}{{(4-2)}^2}=\frac{36}{4}=\frac{9}{1}$