*Question E7D09:
How is battery operating time calculated?*  

*Correct Answer is  C: Capacity in amp-hours divided by average current*  

### AI Response: (may contain errors)  

To calculate battery operating time, we use the formula:

$$
\text{Operating Time (hours)} = \frac{\text{Battery Capacity (Ah)}}{\text{Average Current (A)}}
$$

### Explanation of the Correct Answer (C):
- **Correct Answer: Capacity in amp-hours divided by average current**  
  This formula directly relates the total capacity of the battery (in amp-hours) to the current being drawn (in amps). By dividing the capacity by the average current, you determine how long the battery can supply power before it is depleted.

### Why the Other Choices are Incorrect:
- **A: Average current divided by capacity in amp-hours**  
  This would yield a value in hours per amp-hour, which does not make sense in the context of operating time. It does not provide a measure of how long the battery will last.

- **B: Average current divided by internal resistance**  
  This does not relate to operating time. Internal resistance affects efficiency and voltage drop but does not directly determine how long a battery can operate under a given load.

- **D: Internal resistance divided by average current**  
  This would give a value in ohms per amp, which is not relevant to calculating operating time. It does not provide any useful information about battery life.

### Summary:
- To find out how long a battery will last, always use the formula that divides the battery's capacity (in amp-hours) by the average current (in amps) being drawn. This gives you the operating time in hours.