In Young's double slit experiment, 12 fringes are obtained to be formed in a certain segment of the screen when light of wavelength 600 nm is used. If wavelength of light is changed to 400 nm, number of fringes observed in the same segment of the screen is given by    

18                 

$ n_1\lambda_1 = n_2 \lambda_2 $

$ n_2 = \frac{n_1\lambda_1}{\lambda_2} = \frac{12\times 600}{400} = 18$