A drawing of a cell measures 50 mm across, and the actual cell is 0.05 mm wide. What is the magnification of the drawing?

Magnification $= \dfrac{\text{image size}}{\text{actual size}} = \dfrac{50}{0.05} = 1000$. Both measurements must be in the same unit before dividing; mixing millimetres and micrometres gives the wrong power of ten.