Explain how the applications of inventory management for fixed quantities differ from those for fixed time periods. Give specific instances in which you would use a fixed time period model rather than a fixed quantity model. Provide real-world examples. Identify any challenges you have in setting up an inventory management problem in QM for Windows or Excel and solving it when shortages are an important issue. Explain exactly what the challenges are and why they are challenging. Identify resources that can help you with that.