Exercise 3 solution

ISO 400 is log2(400/100) = 2 steps above ISO 100, so EV = 9 + 2 = 11.
A 100 mm lens: the rule of thumb gives the speed numerically closest to 100, so 1/125 s (1/60 is further away).
This is a rule of thumb for steady hands; stabilisation and enlargement size change it.