A post office is buying stamping machines and they can buy a dual or single model stamping machine. A dual model stamping machine can stamp 50 letters per minute while a single model stamping machine can stamp 30 letters per minute. The dual model stamping machine requires 20 units of glue per minute while the single model stamping machine requires 15 units of glue per minute. Since the single model stamping machine is quieter, the number of single model stamping machines must be more than the number of dual model stamping machines. Further, the post office wants to make sure they can stamp at least 300 letters per minute and use at most 135 units of glue per minute. How many of each stamping machine should they purchase to minimize the total number of stamping machines?