d
Fringe width \(\beta=\frac{ D \lambda}{ d }\)

\(\lambda_{1}=5000 \,\mathring A\)

\(\beta_{1}=\frac{ D }{ d }\left(5000 \times 10^{-10}\right)=5 \times 10^{-4} \,m\)......\((I)\)

\(\beta_{2}=\frac{ D }{(2 d )}\left(6000 \times 10^{-10}\right)= x \text { (let) }\).....\((II)\)

Divide \((II)\) and \((I)\)

\(\frac{\beta_{2}}{\beta_{1}}=\frac{3000 \times 10^{-10}}{5000 \times 10^{-10}}=\frac{x}{5 \times 10^{-4}}\)

\(x=3 \times 10^{-4} m \text { or } 0.3 \,mm\)