The present population of a community is 28000 with an average water consumption of 4200 m³/d. The existing water treatment plant has a design capacity of 6000 m³/d. It is expected that the population will increase to 44000 during the next 20 years. The number of years from now when the plant will react its design capacity, assuming an arithmetic rate of population growth, will be

Arithmetic growth: P = P0 + n*r. Growth rate r = (44000 - 28000) / 20 = 800 per year. Time to reach 60000 (implied capacity) or solve for capacity: 60000 = 28000 + n*800. Wait, the design capacity is 6000 m^3/d. Current consumption is 4200. Growth in consumption = (6000-4200) / (44000-28000) * 28000... actually, just use population: 6000/4200 * 28000 = 40000. 40000 = 28000 + n*800 => n = 15 years.