3. A typical home in the northern U.S. might require 120 MBtu of heat for the average winter. If this heat were supplied by a natural gas furnace operating at 60 percent efficiency, how many cubic feet of gas would need to be purchased? At a cost of $0.90/ccf, what would it cost to heat this house for one season? If a new 80 percent efficient furnace could be installed at a cost of $4,000, how long would it take to pay back the cost of this furnace assuming gas prices remained the same?