Question 1184191
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A carpenter can construct book cases at a cost of {{{cross(P400)}}} $400 each. If
the carpenter sells the book cases for x dollars each, it is
estimated that 300 − 2x book cases will be sold per month.
Express the number of dollars in the carpenter’s monthly
profit as a function of x.
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            Notice that in your post I replaced  P400  by  $400.



<pre>
The monthly profit is the product of the profit per single book case

multiplied by the number of the sold book cases


    PROFIT = P = (x-400)*(300-2x)  dollars.


You can equivalently present it in the form


    P = 300x - 2x^2 - 120000 + 800x  dollars


or in the form


    P = -2x^2 + 1100x - 120000   dollars.
</pre>

Solved and explained.