The Sweet Smell Fertilizer Company markets bags of manure labeled$$ "not less than $60$ lb dry$$ weight." The packaged manure is a combination of compost and sewage wastes. To provide$$ good$-$quality fertilizer, each bag should contain at least $30$ lb of compost but no more than $40$ lb of sewage. Each pound of compost costs Sweet Smell $5$ and each pound of sewage costs $4.$