Let the initial rate be r m/year. In the first 20 years, it retreated 20r meters. In the next 20 years, the rate was 2r, so it retreated 20 × 2r = 40r meters. Total retreat: 20r + 40r = 60r = 1,200 meters. Solving for r: r = 20. Retreat in second 20 years: 40 × 20 = 800 meters. - Groen Casting