Question 1114812
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The function is quadratic; the minimum value is when {{{x = -b/(2a) = 2.6/0.4 = 6.5}}}.<br>
Then just plug that number into the function to find the minimum average cost....<br>
Are you remembering when entering your answer that x is in hundreds?  So the number of dulcimers for the minimum average cost is 650, not 6.5....<br>
And likewise, the minimum average cost is in hundreds of dollars....