 Okay, in 4.6, this is a little bit involved in terms of solving. We are trying to calculate the total heat loss from a house with different surfaces, windows, walls, and the roof. And once we get the total heat that is lost, and that is the heat that we have to buy basically, based on the furnace efficiency of 90%. So we need to calculate heat loss through each of these surfaces, windows, roof, and walls. So let's calculate the heat loss through each of these surfaces. Windows. Do we have all the parameters that are required? Basically area, HDD, and R. Area is given as 580 square feet, and R value is also given as 1. So we know R value. We need to know the HDD. Since the house is located in state college, HDD happens to be 6000, or we can take it as 6000. So we have all three of these. So now heat loss through windows can be calculated by using the formula like area, 580 foot square, times HDD, which is 6000, degrees Fahrenheit days, times 24 hours in a day, divided by R value, which is 1. And if we do this calculation, this foot square, foot square, degrees Fahrenheit, degrees Fahrenheit, hours and hours get canceled, days and days. So we'll be left with BTUs. And in a season we will be losing about 83,520,000 BTUs. So this is only through windows. Now let's calculate through walls, heat loss. So that will be 1,920, 1,920 foot square walls, and HDD is again 6000, and there are 24 hours in a day, which doesn't change, divided by the R value of the walls is given as 19 foot square, degrees Fahrenheit, hour over BTU. So again we can cancel this, and the heat loss from the walls appears to be 14,551,579 BTUs. Now we need to calculate through the roof, heat loss. And we use the same formula, the area is 2,750 foot square, and 6000 degrees days, there are 24 hours in a day, divided by the R value, which is given as 22 foot square, degrees Fahrenheit, hour over BTU. Same things, again, just to make sure these can be canceled, and we get 18 million BTUs. So when you add all these losses up, the windows heat loss, walls heat loss, plus the roof heat loss, we get the total heat loss, or that is the heat that we need to buy, basically the amount of energy that we need to buy, happens to be 116 million, 71,578 BTUs. So if we were to get this heat, or this energy from a furnace that has an efficiency of 0.9, and also the natural gas would provide us 100,000 BTUs in a CCF, so we can calculate the number of CCF, like we have done in the past. We need 116 million BTUs, and when we burn natural gas, we get 100,000 BTUs for every CCF. And also we know that we get only 90% of this energy as output. So although we buy 100,000 BTUs, 0.9 times 100,000 or 90,000 BTUs is what we really get as output. So the CCF that we require total would be 1,289 CCF, or we need to buy 1,289 CCF to supply the heat for this house.