= Solution
The first ideal linear polarizer transmits half of an unpolarized beam. Each subsequent polarizer is $60^\circ$ from the preceding transmitted <polarization>, so <Malus's law> gives
$$
\boxed{\frac{I_{\rm out}}{I_{\rm in}}=\frac12\cos^2(60^\circ)\cos^2(60^\circ)=\frac1{32}.}
$$
Intensities, rather than coherent field amplitudes for different input <polarization> components, are averaged for the unpolarized beam. The output is polarized along the last polarizer's axis.
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