Final answer:
In the measurement 20.4 cm, the '4' in the tenths place is the uncertain digit as it is the last and estimated part of the measurement.
Step-by-step explanation:
In the measurement 20.4 cm, the uncertain digit is the 0.4 or specifically the '4' in the tenths place. This is because, in measurement, the last digit is always the first uncertain digit, as it's the one estimated by the person performing the measurement. This is due to the precision of the measuring tool, determining how many significant figures you can confidently report. The first two digits, '2' and '0', are considered certain because they fall within the graduations of the measurement tool and are clearly identified. But the '4' has been estimated between two graduations. Therefore, it represents the estimated part of the measurement, which conveys the precision of the measuring tool and the observer's estimation.
The uncertain digit is the one that has been estimated or is uncertain due to the limitations of the measuring tool. In this case, we can be certain about the first two digits (2 and 0), but the last digit (4) is uncertain. It should be noted that when we express measured values, we can only list as many digits as we initially measured with our measuring tool. The last digit is usually the first digit with some uncertainty.