According to the U.S. Department of the Interior, the amount of energy lost because of poorly insulated homes is equivalent to 2 million barrels of oil per day. In 2009, this is more oil than the United States imports from Saudi Arabia each day. If we were to [insulate our homes as determined by Example 2-7, we could eliminate our oil dependence on Saudi Arabia. If the cost of electricity increases to $0.15 per kWh and the cost of insulation quadruples, how much insulation should be chosen in Example 2-7?