 Solution

With the proximity-resolution convention $w=k\sqrt{\lambda g}$, the maximum mask-wafer gap isFor $w=5\mu \mathrm{m}$ and $k=2$, $g_{\max }\simeq 17.1\mu \mathrm{m}$ at $365\mathrm{nm}$. If the g-line is used instead, $g_{\max }\simeq 14.3\mu \mathrm{m}$ at $436\mathrm{nm}$. The wavelength must be specified; the question's preceding choice of two lamp lines does not select a unique number. This estimate neglects extra loss of resolution from finite resist thickness, mask imperfections and angular illumination spread.