Signal Company is a medium-sized manufacturer of computers which are either sold locally or exported. The company has contracts to provide at least four computers per month locally and at least six computers per month for export, and it has the capacity to produce no more than 20 computers per month. The costs involved in producing a computer locally are $3,000 and for each exported computer are $4,000; a maximum of $72,000 is available per month for production costs. Let the number of computers produced per month for local sale be x, and the number produced per month for export be y. Write a set of inequations for the information given. Identify the constraints.