According to the MWRA, the "safe yield" of the water system is 300 million gallons per day. In the 1980s, the demand on the system averaged around 330 million gallons per day. At the 1980s rate of demand, how many years of supply do the full Quabbin reservoir, which is 412 billion gallons, and Wachusett reservoir, which is 65 billion gallons represent