The fringe width $\beta = \frac{\lambda D}{d}$. The number of fringes $n$ in a segment of length $L$ is $n = \frac{L}{\beta} = \frac{Ld}{\lambda D}$. Since $L, d, D$ are constant, $n \propto \frac{1}{\lambda}$. Thus, $n_1 \lambda_1 = n_2 \lambda_2$. $8 \times 600 = n_2 \times 400$, so $n_2 = \frac{4800}{400} = 12$.