In a Young's double slit experiment, a student observes 8 fringes...

Wavelength of light used is inversely proportional to number of fringes observed.

Hence, we can write:

${n}_{1}{\lambda}_{1} = {n}_{2}{\lambda}_{2} ⇒ {n}_{2} = \dfrac{{n}_{1}{\lambda}_{1}}{{\lambda}_{2}} = \dfrac{8 \times \left(600 \times {10}^{- 9}\right)}{\left(400 \times {10}^{- 9}\right)} = 12$