Answer
At about $x=60,000$ units produced.
Work Step by Step
Marginal cost is defined as $f'(x)=\displaystyle \frac{dc}{dx}.$
$f'(x)$ increases where $f''(x) \gt 0 $ and decreases where $f''(x) \lt 0$ .
The left part of the graph is concave down, $f''(x) \lt 0$,
and at approximately $x=60$ there is an inflection point, where concavity changes to up, $f''(x) \gt 0.$
Thus, marginal cost $f'(x)$ changes from decreasing to increasing at about $x=60,000$ units produced.
