A supermarket selling a certain product during the next three months estimates the demands for the next three months as 500, 600 and 400, which should be met on time. The supermarket obtains this product from two different suppliers. The maximum number of products that these suppliers can provide is 400 units per month for each supplier. The purchase cost of the product from the suppliers changes from one month to another as follows;

  Supplier    Month 1( $/unit) Month 2 ($/unit) Month 3 ($/unit)

   1 100    110    120

   2 115    108    125

The supermarket can purchase more than is needed for a month and store the surplus to satisfy the demand in a later month. The cost of keeping a unit of the product in the storage is $5 per month. Formulate an appropriate LP.