Answer
$d=36cm$
Work Step by Step
We know that:
$\theta=\frac{y}{L}=1.22(\frac{\lambda}{d})$
This can be rearranged as:
$d=1.22\frac{L\lambda}{y}$
We plug in the known values to obtain:
$d=1.22\times \frac{160\times 10^3\times 550\times 10^{-9}}{0.30}=36\times 10^{-2}m=36cm$
