The radius of first half period zone is $0.05cm$  and $\lambda =2500A^0$ .  The distance between slit and screen is

Radius $r=0.05cm=0.05\times {10}^{-2}m$

Wavelength $\lambda =2500A^0$

Distance between slit and screen $b=?$

       $r_n=\sqrt{nb\lambda }$

$\Rightarrow b=\frac{r^2}{n\lambda }$

      $=\frac{{\left(5\times {10}^{-4}\right)}^2}{1\times 2500\times {10}^{-10}}$            

      $=\frac{25\times {10}^{-8}}{25\times {10}^{-8}}$

$\therefore b=1m$