please solve step by step and don't use any program

Consider a version of the warehouse-location problem, where a manager must decide which of N warehouses to open for meeting the demands of M customers for a good with minimum costs. The decisions to be made are which warehouses to operate and how much to ship from any warehouse to any customer. The relevant parameters are: f; = Fixed operating cost for warehouse i, if opened Cij = per unit transportation cost for shipping from warehouse i to customerj D; = Demand at customer j a. Formulate the problem as a mixed integer programming model. b. Assume that each customer has to be served from a single warehouse; i.e., no splitting of orders is allowed. How should the warehousing model be modified? C. Indicate how the model would be modified if there were three distribution stages (facilities->warehouses->customers). Consider a version of the warehouse-location problem, where a manager must decide which of N warehouses to open for meeting the demands of M customers for a good with minimum costs. The decisions to be made are which warehouses to operate and how much to ship from any warehouse to any customer. The relevant parameters are: f; = Fixed operating cost for warehouse i, if opened Cij = per unit transportation cost for shipping from warehouse i to customerj D; = Demand at customer j a. Formulate the problem as a mixed integer programming model. b. Assume that each customer has to be served from a single warehouse; i.e., no splitting of orders is allowed. How should the warehousing model be modified? C. Indicate how the model would be modified if there were three distribution stages (facilities->warehouses->customers)