Question 181439
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Let x = number of days until shipped
then
f(x) = (60-2n)(400 + 20n)
where f(x) is the income 
.
f(x) = (60-2n)(400 + 20n)
f(x) = 24000+1200n-800n-40n^2
f(x) = -40n^2+400n+24000
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Since the coefficient for the n^2 term is -40 (negative), we know that it is a parabola the opens downward.  So finding the axis of symmetry will give us the max.
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axis of symmetry = -b/2a = -400/2(-40) = -400/-80 = 5
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Answer: 5 days