Answer
On $(0,2)$ and $(5,9)$, marginal revenue increases.
On $(2,5)$ and $(9,12)$, marginal revenue decreases.
Work Step by Step
Marginal revenue is defined as $r'(t)=\displaystyle \frac{dy}{dt}.$
$r'(t)$ increases where $r''(t) \gt 0 $ and decreases where $r''(t) \lt 0$ .
$r''(t) \gt 0$ happens on intervals whre $r$ is concave up, and
$r''(t) \lt 0$ happens on intervals whre $r$ is concave down.
We read from the graph approximate inflection points:
t=2,5, and 9.
On $(0,2)$ and $(5,9)$, the graph is concave up, so marginal revenue increases.
On $(2,5)$ and $(9,12)$, the graph is concave down, so marginal revenue decreases.
