Answer:
 see attached
Step-by-step explanation:
The two inequalities you want to plot are ...
 x + y ≤ 75 . . . . . . . . the auditorium can hold no more than 75 people
 5x +7.5y ≥ 450 . . . a minimum of $450 revenue is needed to cover costs
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These are probably most easily plotted by graphing their intercepts. The boundary line for the first inequality will have x- and y-intercepts of 75. Shading will be below the line.
The boundary line for the second inequality will have an x-intercept of 450/5 = 90, and a y-intercept of 450/7.5 = 60. Shading will be above the line.
On of the points in the potential solution area is shown in the attached graph:
 (x, y) = (25, 50) . . . . fills the theater, gives $500 revenue