Answer
$\frac{1}{125}s$
Work Step by Step
Example 25–2 shows that the exposure is proportional to the product of the lens opening area and the exposure time.
Page 715 discusses how the square of the f-stop number is inversely proportional to the lens opening area.
The f-stop number has been doubled from 5.6 to 11, which decreases the area by 4.
The exposure time will have to increase by a factor of 4. To keep the picture properly exposed, the new exposure time should be $4(\frac{1}{500}s)=\frac{1}{125}s$.
