$I\propto a^2$

At maxima ${\mathrm{a}}^{\mathrm{\prime }}={\mathrm{a}}_1+{\mathrm{a}}_2$
At maxima ${\mathrm{a}}^{\prime \prime }={\mathrm{a}}_1-{\mathrm{a}}_2$
Given that relative intensities are 400 : 1 r'.e., amplitudes arc in the ratio 20 : 1
Ratio of intensity at maxima =$\frac{(20+1{)}^2}{(20-1{)}^2}$

$=\frac{(21{)}^2}{(19{)}^2}\approx \frac{11}{9}$