Exercise 1 solution

Magnification m = 25 / 50 = 0.5.
Effective f-number = 4 x (1 + 0.5) = f/6.
Light lost = log2((1.5)^2) = log2(2.25) = 1.17 stops (about 1 1/6 stops; the stops conversion is my own application of Chapter 1, check).