hey .........look.................

 

no. of waves=  t(thickness)/

therefore.............no. of waves is same in both..........

now

=  v  / f  = c/u f      [ u= refractive index]

no. of waves inA=  t  / ( c/u_Af)           (  c---- speed of light

                                                             f-----frequency  ....remains const)

thickness of B= t/3

thickness of C= 2t/3  (   as in the ratio of 1: 2)

therefore............

no. of waves in B  and C =  (t/3)/  ( c/u_bf)  +    (2t/3)/ (c/u_cf)

equating them............we get......................

and cancelling tf/c...................

we get