Answer
$g$ is greater or equal to $30$ between $40mi/h$ and $50mi/h$
$40 \leq v \leq 50$
Work Step by Step
We have a key point $g=30mi/gal$. We have to find speed interval where the gas consumption is $30mi/gal$ or better. It means, that we have to find interval where $g\geq 30$.
$10+0.9v-0.01v^2\geq30$
$30=10+0.9v-0.01v^2$
$-0.01v^2+0.9v-20=0$
$v^2-90v+2000=0$
$v_1=40$ ; $v_2=50$
We have $3$ possible intervals $(-\infty, 40]$ ; $[40, 50]$ ; $[50, +\infty)$
In the first interval (till $40$) it is negative. In the second interval it is positive. In the third interval it is negative.
We need the interval where it is greater or equal to $30$.
$g$ is greater or equal to $30$ between $40mi/h$ and $50mi/h$
