Step-by-step explanation:
a) Since the precision of the ruler is 1 mm, the uncertainty associated with this ruler is +-0.5mm. So the tire tread has a depth of 7.0 mm +- 0.5mm. The amount of tire thread that has worn off is
11.2 mm - (7.0 mm +- 0.5 mm) = 4.2 +- 0.5 mm
b) The tire thread can anywhere between 3.1 mm and 3.5 mm. Since the lower limit has gone below the legal limit of 3.2 mm, it's time to change the tire.
