Here, we are comparing two alternatives. The alternatives are 1> Wood Frame 2> ICF
Bill is lesser for the ICF but the construction cost of ICF is high.
The ICF costs $150x2100=$315,000 for a typical home which is 10% higher. So, it costs $31,500 more. This amount is to be recovered.
The monthly bill is $210 which is 60% lower. Thus, x=210/0.6=$350 will be the monthly bill for the wood frame. It is $140 more.
So, it will take 31500/140=225 months to recover the cost.
Focus on the difference between feasible alternatives (Principle 2)l Insulated concrete forms (ICF) can be used...