An environmentally conscious customer hires a contractor to re-build his garage cement floor at minimum cost. The contractor is trained to used 3 main types of recycled materials when making eco-friendly cement: ash, dust and slag. Each cubic foot of ash, dust and slag costs $2, $3 and $5 respectively. In order to build eco-friendly cement, the amount of dust plus twice the amount of slag used should be at least 50 cubic feet for small jobs like this one. Moreover, the amount of dust used should not exceed the combined amount of ash and slag used. What mixture of materials should the contractor use to produce exactly 300 cubic feet of eco-friendly cement for this job?