= Solution
A fully clear opening requires the higher clearing threshold, not merely the onset of thinning. At $\tau=20\,\mathrm s$,
$$
I_{\rm clear}=\frac{D_{100}}{\tau}=\frac{50}{20}=2.5\,\mathrm{mW\,cm^{-2}}.
$$
The intensity curve crosses this level at about $x=0.25$ and $3.75\,\mu\mathrm m$. Therefore the clear opening is approximately
$$
\boxed{w\simeq3.75-0.25=3.5\,\mu\mathrm m.}
$$
The longer exposure widens the opening because even the relatively dim image tails now accumulate enough <exposure dose> to clear the positive <photoresist>. The edge readings are approximate; interpolating a plotted curve does not support nanometre precision.
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