30,000 people consume 1200*30000*365/4 = 3285*10^6 liters of water per year = 3285*10^3 cubic meters of water per year. I divided by 4 accounting for the fact that the average family consists of 4 people. To find the depth the lake would lose per year, divide this volume by the area of the lake which is equal to 64 square kilometers = 64*10^6 square meters, and you will get= . = 0.051 m = 5.1 cm.