interference ques

q: Sodium light has 2 wavelengths; lambda1=589nm and lambda2=589.6nm.As the path difference inc, when is the visibility of the fringes maximum??

Ans- 0.29mm..

if i find a position where bright fringes of both will coincide other than central maximum.

how to decide d ans--

.15mm, .30mm, .45mm,....all will do..
for n1= 589.6*5, 589.6*10,... taking lambda=5890..

i shud hav marked .15mm, right?
but ans given is Ans- 0.29mm..

please help
thanks