The question is incomplete, I came across the same question and here is the complete one:
Mary is considering upgrading the insulation in her house. Contractor suggests upgrading the insulation of her roof from R-16 to R-28. The estimate for this job from the contractor is $800. Her house is located in Pittsburgh, PA (HDD = 6,000) with a roof area of 1926 ft2. Mary’s home is heated with natural gas. Last year, the average price she paid for natural gas was $9.9/MCF. Assuming an AFUE of 81.2%, how long will it take Mary to recover her investment (in years)?
The answer is 8.83 years. Price of insulation = 800 / 1926 = 0.4153 $/sq.ft Cost of energy = 9.9 / 1,000,000 = 0.0000099 $/BTU Duration to recover investment = (0.4153 x 16 x 28 x 0.812) / (0.0000099 x (28 - 16) x 6000 x 24) = 8.83 yrs