Jack is buying a furnace. He has two choices. A used one for $840 will last for 3 years: and a new one for $2100 will last for .5 years. Both will have scrap 1lralues of zero. Jack uses a MARR of 3%. If Jack decides to purchase the used furnace, how much does he save on a present worth basis compared to buying the new furnace? {Use PW analysis with repeated-lives. Assume the annual operating costs of the furnaces are the same}