Answer
a. 0.1 mi
b. 0.05 mi
Work Step by Step
For the measurement $15.0~mi$, we have
a. The precision is 0.1 mi (because the last digit $0$ is in the $0.1$ place).
b. The greatest possible error is one-half the precision:
$$0.5\cdot 0.1 \ mi=0.05 \ mi.$$
