Question 1165912
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They just explained everything to you in the post.   You only need to combine 
their explanations in one simple formula,  and then calculate


        profit   P = R - C = 5x - (x + 4000) = 5*1000 - (1000+4000) = 5000 - 5000 = 0.


The profit is  0  (zero)  at the given conditions.


So,  x = 1000  is the break point in this problem.