Discussion
A screen has \( 4 \) mm round apertures on a \( 6 \) mm square pitch. Its open area fraction is about
Answer: B
Correct answer: 34.9 per cent
Take one hole and the square of pitch that belongs to it:
\[ \frac{\pi r^2}{p^2} = \frac{\pi (2)^2}{6^2} = \frac{12.57}{36} = 0.349 = 34.9\ \mathrm{per\ cent} \]
The distractor \( 44.4 \) squares the diameter ratio without the factor for a circle inscribed in a square. Open area governs capacity directly, which is why perforations are set as close as the sheet's strength allows - and why blinding, by reducing the effective open area, hurts capacity so quickly.
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