Answer
a. 11.
b. 2.1 ms.
Work Step by Step
a. The f-number of a lens is the ratio of its focal length to its diameter. It is an integer.
$$\frac{180.0mm}{16.36mm}=f/11$$
b. The amount of light passing through the lens during the exposure time should remain the same.
Moving from f/11 to f /2.8 is four steps of 2 in intensity, so one needs 1/16 of the exposure time.
In other words, the area increases by a factor of 16. To let in the same amount of light, the exposure time decreases by a factor of 16.
$$\frac{1}{16}\frac{1}{30}s=\frac{1}{480}s=2.1ms$$