Exercise 3 solution

Field of view: 200 mm x 2 = 400 mm full-frame equivalent.
Handheld rule on the actual focal length: the speed numerically closest to 200 is 1/250 s (1/125 is further away). The article's own example gives "no slower than 1/200 s" for 200 mm, so 1/250 s is the nearest standard speed at or above that.
Unsure about: whether the rule should use the equivalent focal length (400 mm, which would suggest 1/500 s) rather than the real one. Not confirmed in the sources read (check). Being safe, use the faster speed.