Final answer:
When viewing a paramecium under a microscope at 40x magnification, its length would appear about 1 mm.
Step-by-step explanation:
The measurements of paramecium length through a microscope at 40x magnification can vary. However, let's assume that the length of the paramecium is approximately 25 μm. When viewing the paramecium under a microscope at 40x magnification, its length would appear 40 times larger than its actual size. Therefore, the paramecium would appear to be about 1000 μm or 1 mm in length.