Final answer:
The percentage uncertainty remains unchanged when you multiply a measurement by a constant because the constant introduces no additional uncertainty to the calculation.
Step-by-step explanation:
When you multiply a measurement by a constant, the percentage uncertainty of the measurement does not change. This is because the constant itself has no uncertainty and thus does not contribute to the uncertainty of the result. For example, if you have a length measurement with a 2% uncertainty and you multiply that length by a constant factor to find an area, the resulting area will also have a 2% uncertainty. This principle applies in cases where measurements are involved, typical in physical sciences and experimental results.