New Hospice is building an extension. It can use single-pane glass windows that cost $100 each or it can buy double-pane windows that cost $150 each. The windows should last 20 years before they have to be replaced. The hospice expects that heating costs will be lower by $5 per year for each double-pane window. The hospice can borrow or invest money at 10%. Which type of window should it buy? What would your answer be if the interest rate was 6%? Why did changing the interest rate change the answer?