Total number of waves = \(\frac{{(1.5)t}}{\lambda }\) ....\((i)\)
\(\because\) \(\left(\begin{array}{l}{\rm{Total\,number}}\\\;\;{\rm{ \,of\, waves}}\end{array} \right) = \left({\frac{{{\rm{optical \,path \,length}}}}{{{\rm{wavelength}}}}} \right)\)
For \(B\) and \(C\)
Total number of waves = \(\frac{{{n_B}\left({\frac{t}{3}} \right)}}{\lambda } + \frac{{(1.6)\left({\frac{{2t}}{3}} \right)}}{\lambda }\) ....\((ii)\)
Equating \((i)\) and \((ii)\) \({n_B} = 1.3\)