Final answer:
To determine how many red blood cells can fit across a grain of salt, we convert the width of the salt (0.5 mm) to micrometres (500 µm) and divide by the diameter of a red blood cell (8 µm), resulting in 62.5 red blood cells fitting across the grain of salt.
Step-by-step explanation:
If a grain of salt is 0.5 mm wide and a typical human red blood cell is about 8 micrometres (8 x 10-6 m) in diameter, we can determine how many red blood cells can fit across one grain of salt by converting the width of the grain of salt to micrometres and then dividing by the diameter of a red blood cell.
Firstly, convert the 0.5 mm to micrometres: 0.5 mm = 500 micrometres (since 1 mm = 1000 micrometres).
Now we divide the width of the grain of salt by the diameter of a red blood cell:
500 micrometers / 8 micrometers = 62.5.
Therefore, 62.5 red blood cells can fit across a grain of salt that is 0.5 mm wide, assuming red blood cells can pack perfectly without any gaps.