His initial cost is $28,000. Every decade the value of the house doubles. We are in year 1975. Let's add 10 years (a decade) 1975+10= 1985. Since we added ten years, we will double $28,000 which is $56,000. Let's add 10 more years 1985+10 is 1995. Since we added 10 years more, we double $56,000 which is $112,000. Now we can still add 10 more years. Do the same thing you did for the first and second decades. You should get the year 2005 and $224,000. We can still add 10 more years to get to 2020. Our year will be 2015 and our total amount in dollar is $224,000² = $448,000. Since the value doubles every decade you ignore the 5.
