 Okay, this is 5.11, here Dave has a house and he is trying to improve the insulation and the contractor comes up with an estimate of $775, he is using natural gas to heat the home and we are trying to calculate how long will it take Dave to recover his investment. And this is a typical problem that uses an equation for payback period equal to CI, cost of insulation times R1 times R2 times efficiency divided by cost of energy times again R2 minus R1, this is the final R value minus initial R value times HDD times 24. Do we have all the pieces of information for this formula? Cost of insulation, cost of insulation it should be in dollars per square foot. So Dave has got an estimate of $775 to cover an area of 1200 square foot, roof area happens to be 1200 square foot. To cover this area with insulation it costs $775, so we can calculate the cost per square foot. This happens to be 65 cents per square foot. That's what we want per square foot. Do we know R1? Yes we know, R1 equal to 16, R2 equal to 22 and the furnace efficiency E is equal to 0.86 and we also know HDD. HDD is 6000, we know 24 hours. Only thing here is we need to calculate the cost CE, cost of energy. And we are paying here or Dave is paying $9.86 per MCF. Remember MCF has is basically 1 million BTUs. So the price would be $9.86 per million BTUs. So this would be equal to 0.00000986. If we plug this into this formula here, 0.65 times 16 times 22 times 0.86 divided by, this is 0.00000000986. R2 minus R1 happens to be 6 and 6000 is the degree days and 24. This should come out to be I guess 23 years roughly. This is kind of a long time period to recover the investment.