A camera lens has a focal length of 180.0 mm and an aperture diameter of 16.36 mm

A camera lens has a focal length of 180.0 mm and an aperture diameter of 16.36 mm.
(a) What is the f-number of the lens?
(b) If the correct exposure of a certain scene is 1/30s at f/11, what is the correct exposure at f/2.8?