Exercise 1 solution

DOF = 2 * u^2 * N * c / f^2 with u = 3000 mm, f = 35 mm, c = 0.03 mm.
f/4: 2 * 9,000,000 * 4 * 0.03 / 1225 = about 1763 mm = 1.76 m
f/8: 2 * 9,000,000 * 8 * 0.03 / 1225 = about 3527 mm = 3.53 m

Doubling the f-number doubles the DOF (3.53 / 1.76 = 2), matching the proportional rule. The value of c (0.03 mm) is a convention not confirmed by the source (check).