 OK, this problem 4.7 is very similar to what we have done in 4.6, which is calculating the heat loss from various surfaces and adding up all those to get the total heat loss from the house and to calculate the oil requirement based on the furnace efficiency. The house is located in Bismarck, North Dakota, and HDD there is given as 9,000. So HDD is given. This HDD remains the same for all different surfaces. And we also know the surfaces of each of these components, like windows. Let's do the calculation first for windows. Windows area is given as 860 square feet. And we also know the R value of this. R value is given as 1. This is foot square degrees Fahrenheit hour over BT use. So we can calculate heat loss through windows. All we need is the area, 860 foot square times HDD, which is 9,000 in this case, times 24 hours in a day divided by we get the R value here. That is 1 foot square degrees Fahrenheit hour over BTU. So foot square, foot square, cancel degrees Fahrenheit, degrees Fahrenheit, 24 hours, 24 hours, days and days. So the total heat loss through windows is the number that we get here. This happens to be 185,760,000 BTUs. Similarly, we can calculate the heat loss. This is through windows. We can calculate heat loss through the walls. Wall area is 2,920 foot square times 9,000 degrees Fahrenheit days times 24 hours per day divided by R value of 19 for walls. And we can cancel out the units and make sure that everything makes sense. And this turns out to be 33,195,789 BTUs. Now, heat loss through the roof. We need to calculate it again separately. And the area of the roof happens to be 3850 square feet and 9,000 degrees days times 24 hours over a day divided by the roof generally has higher R value, 22 foot square degrees Fahrenheit hour over BTUs. Now, let's cancel out these units. And the heat loss appears to be 37,800,000 BTUs. So the total heat loss is the sum of all these three. And when you add these up, you get 256,755,789 BTUs. Now, the furnace efficiency is given. And we are using, in this case, heating oil. Heating oil, 130,000 BTUs. So how much is required? How much heating oil is required? We need 256,755,789 BTUs. When we buy heating oil, we get 130,000 BTUs for every gallon. And although we get 130,000, this is theoretical. The efficiency is given as 0.8. So only 80% of this will be really available as heat. So that makes it actually the requirement a little bit higher. So that is equal to now. If we do this calculation, it will be 2,468.8 gallons. Or 2,4689 roughly gallons of oil is required to heat this place.