If 10⁹ electrons leave a body every second, approximately how long is needed for the remaining body to develop a net positive charge of 1 C?

Per second the body loses 10⁹ × 1.6×10⁻¹⁹ = 1.6×10⁻¹⁰ C. To net 1 C → 1 / 1.6×10⁻¹⁰ ≈ 6.25×10⁹ s, which divided by (365 × 24 × 3600 ≈ 3.15×10⁷) gives roughly 200 years. Worked out as Example 1.1 in the chapter.