What is Delphi solving technique

Jenny​ walters, who owns a real estate​ agency, bought an old house to use as her business office. she found that the ceiling was poorly insulated and that the heat loss could be cut significantly if six inches of foam insulation were installed. she estimated that with the​ insulation, she could cut the heating bill by​ $80 per month and the​ air-conditioning cost by​ $70 per month. assuming that the summer season is three months​ (june, july, and​ august) of the year and that the winter season is another three months​ (december, january, and​ february) of the​ year, how much can jenny spend on insulation if she expects to keep the property for five​ years? assume that neither heating nor​ air-conditioning would be required during the fall and spring seasons. if she decides to install the​ insulation, it will be done at the beginning of may.​ jenny's interest rate is​ 6% compounded monthly.