What is the filter area for a 25 mm cassette?

• A. 1234 mm^2
• B. 490 mm^2
• C. 0.00785 mm^2
• D. 855 mm^2

Answer: (D)

The main idea here is that the filter area is the area of the circular opening air passes through, found with A = πr^2. If the opening is about 33 mm in diameter, the radius is 16.5 mm. Then the area is π × (16.5 mm)^2 ≈ 3.1416 × 272.25 ≈ 855 mm^2. So the filter area comes out to roughly 855 square millimeters. This matches the value shown. The other numbers would correspond to much smaller or larger opening diameters than the 33 mm assumed here, so they don’t align with the actual opening size used in this context.