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The cost of producing $x$ gadgets is given by

$$C(x) = 10+5x+\dfrac{1}{60}x^3$$

The revenue equation is

$$R(x) = 90x-x^2$$

For what quantities is the profit an increasing function of $x$? In other words, when is the marginal profit positive?

$x<70$

$x>30$

$ 30 < x < 70$

$ 0 < x < 30 $