Real size under the microscope

A microscope picture shows a cell 32 mm wide, at a magnification of 400 times. You will write two functions that turn sizes in a picture into real sizes and back, and find how many red blood cells would fit across this cell.

The idea. Size in the picture = actual size × magnification. 1 mm = 1000 µm.

Your tasks.

1. In actual_um, return the actual width in micrometres.
2. In picture_mm, return the width in the picture in millimetres.
3. Set fits to how many whole red blood cells, 7 µm wide, fit across the cell.