The image size would be 1.6 mm.

Quick Scoop

To find the image size, you use the standard magnification formula from school biology:

magnification=image sizeactual size\text{magnification}=\frac{\text{image size}}{\text{actual size}}magnification=actual sizeimage size​

Rearranging this gives:

image size=magnification×actual size\text{image size}=\text{magnification}\times \text{actual size}image size=magnification×actual size

Here:

  • Actual size of the cell = 0.04 mm.
  • Magnification = 40.

So:

image size=40×0.04 mm=1.6 mm\text{image size}=40\times 0.04\text{ mm}=1.6\text{ mm}image size=40×0.04 mm=1.6 mm

You’ll also find this exact question answered with 1.6 mm as the correct result in online homework/quiz solutions for magnification problems.

Key fact in one line

If a cell measured 0.04 mm is magnified ×40, the drawing or image on paper/screen should be 1.6 mm long.

Information gathered from public forums or data available on the internet and portrayed here.