In which part of a cover letter does the applicant explain the purpose of the letter?

Suppose that a paving company produces paved parking spaces (q) using a fixed quantity of land (t) and variable quantities of cement (c) and labor (l). the firm is currently paving 1,000 parking spaces. the firm's cost of cement is $3 comma 600.003,600.00 per acre covered (c) and its cost of labor is $35.0035.00/hour (w). for the quantities of c and l that the firm has chosen, mp subscript upper c baseline equals 60mpc=60 and mp subscript upper l baseline equals 7mpl=7. is this firm minimizing its cost of producing parking spaces?