Please I need the solution of this question with explain the details also with clear graph:

An insurance company employs full and part-time staff, who work 40 and 20 hours per week, respectively. Fulltime staff are paid $800 per week and part-time staff $320. In addition, it is company policy that the number of part-time staff should not exceed one-third of the number of fulltime staff. If the number of worker-hours per week required to deal with the company's work is 900, how many workers of each type should be employed in order to complete the workload at minimum cost?