Lens E is marked 10× and lens O is marked 40×.
A cell is viewed through lenses E and O.
The image of the cell is 0.8 mm in diameter.
What is the actual diameter of the cell?

Moriarty
Total magnification of 400x which means that the image you are seeing is 400 times the actual size. This "magnified" image is 0.8mm so the "original/actual" image must be 400 times smaller than this (0.8mm/400).

Niamhoc22
because you are looking through both lenses the total magnification is 400 (10x40). At magnification 400 the cell has a diameter of 0.8mm, so to find what the diameter is with no magnification you must divide 0.8 by 400. :)

what.the.matter.is.the
To find the magnification of a cell you multiply the lens powers, here it is 10x and 40x. Therefor, the final magnification will be 400x ( See x as magnified 400 times).
Now 400times of an object makes it 0.8mm in diameter.
That is,
400y = 0.8;
y = 0.8/400
y = 0.002mm

Me
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