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12-73.

Only consider orders ≥ 80 chairs.

The cost of the chairs before the discount is 145x for x chairs.

The discount for 81 chairs is 0.50(1)(81).
The discount on 82 chairs is 0.50(2)(82).
The discount on 83 chairs is 0.50(3)(83).
...

The discount amount is 0.50(x – 80)x.

Write an equation for the total cost, including the discount.
Then maximize the equation.
Remember, x = # of chairs and the problem asks for the cost.