a. What is the potential energy contained in the water in a lake that has a surface area of 5 square kilometers (km²), an average depth of 20 meters (m), and an elevation of 25 m above the hydroelectric power plant? Water has a density () of 1,000 kg/m³.

b. If the hydroelectric plant produces 500 MW how long before the plant runs out of water? Assume the conversion efficiency of potential energy to electrical energy is 85%.