 OK. This is 5.10. Heating degree days are given. In this problem, basically, we're trying to calculate the payback period again using the formula that we have, that we derive, basically. And payback period for adding insulation is cost of insulation times r1 times r2 times efficiency divided by cost of energy times the difference in our value times HDD times 24. That is the formula that we need to use. We have all the data. HDD is given as 4,600. And cost of insulation CI is given as 60 cents per square foot. And the efficiency of the furnace is given as 78% or 0.78. We know the initial r value, r1, 13. And we know the final r value or improved r value, which is 22. The difference is 9. And we know the price of energy or cost of energy. We're paying $1.13 to buy a gallon of oil. And when we buy a gallon of oil by paying $1.13, we get 130,000 BTUs. So for every BTU, we pay 0.000869 dollars. So we can use the formula now. It consists of CI. CI is cost of insulation, which is 0.06 per square foot, dollars per square foot times r1, which is 13. Again, units, you have to make sure foot squared degrees Fahrenheit over BTU. And here r2 is 22 times the efficiency, which is 0.78, divided by the cost of energy, which is dollars per BTU. 0.0000869 per one single BTU times the difference between these two r values here. And that is 22 minus 19, 9 times degree days. Heating degree day is 4,600 times 24. And when you do this calculation, it turns out to be 15.5 years is the payback period.