Question

The average cost when producing x items is found by dividing the cost function, C(x), by the number of items,x. When is the average cost less than 100, given the cost function is C(x)= 20x+160?

A) ( 2, infinit)
B) (0,2)
C) (-infinit,0) U (2,infinit)
D) (- infinit,0] U [2,infinit)

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Answer 1

9514 1404 393

Answer:

A) (2, ∞) . . . . or C) (-∞, 0) ∪ (2, ∞) if you don't think about it

Explanation:

We want ...

C(x)/x < 100

(20x +160)/x < 100

20 +160/x < 100 . . . . . separate the terms on the left

160/x < 80 . . . . . . . subtract 20

160/80 < x . . . . . multiply by x/80 . . . . . assumes x > 0

x > 2 . . . . . . simplify

In interval notation this is (2, ∞). matches choice A

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Technically (mathematically), we also have ...

160/80 > x . . . . and x < 0

which simplifies to x < 0, or the interval (-∞, 0).

If we include this solution, then choice C is the correct one.

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Comment on the solution

Since we are using x to count physical items, we want to assume that the practical domain of C(x) is whole numbers, where x ≥ 0, so this second interval is not in the domain of C(x). That is, the average cost of a negative number of items is meaningless.