ab 1. (16p) A family wants to replace their 25 year old gas-fired combi boiler (24 kW) where the unit cost of natural gas is 0.15/kWh. The current combi has 60% efficiency and operated for (a+2) hours a day for 3 months each year. The family wants to make a decision whether to buy a new combi with 90% efficiency and 4000 cost. Give your suggestion to this family whether to buy the new one or keep the current one considering a simple payback period of 5 years