Final answer:
The black hand of a duplex air gauge, which indicates the air pressure in a system, cannot be determined to read a specific value without additional system context. Hence, the answer is 'Cannot be determined.'
Step-by-step explanation:
The question pertains to the reading on a duplex air gauge during normal operation of a machine or vehicle that utilizes a compressed air system. In automotive and machinery context, a duplex air gauge is typically used to monitor the pressure within the air system. The black hand on such a gauge usually indicates the actual air pressure in the system at any given moment.
For a gauge operating in normal service conditions, the reading of the black hand can vary significantly depending on the specific requirements of the equipment it is installed on. However, if the gauge is calibrated in psi (pounds per square inch), common readings might be within a range, such as between 24 to 115 psi. Without additional context or specifications about the system, it is not possible to determine a single value that the black hand should read.
Therefore, the correct answer is option 4: Cannot be determined. This is because various systems will require different operating pressures for normal functionality, and there isn't a one-size-fits-all pressure value.