#### Answer

a. There are three significant figures in the measurement. The three digits in the measurement (31.8) are all significant because they are all non-zero numbers.
b. There is uncertainty in the last digit of the measurement. After an exact number, we are always supposed to estimate the next digit. This last digit always has uncertainty. In this case, we know that the meniscus of the liquid in the graduated cylinder is above the 31-mL mark and just below the 32-mL mark. An extra digit is then added; because the meniscus is much closer to 32 mL than it is to 31 mL, 31.8 seems to be a good estimate.