Ok, so
First of all, we got that the wound was measured to be 0.8cm.
This measurement equals to:
[tex]0.8\operatorname{cm}\cdot\frac{10\operatorname{mm}}{1\operatorname{cm}}[/tex]0.8cm is equal to 8 millimeters.
Now, the greatest possible error in a measurement is one half of the precision (smallest measured unit).
8 mm was measured to the nearest 1 mm, so the measuring unit is 1 mm.
So, one half of the precision (1mm) is 0.5
Therefore, the greatest possible error is 0.5 mm
