#### Answer

The extremes are $12$ and $20$. The means are $m$ and $18$.
$m = \frac{40}{3}$

#### Work Step by Step

A proportion takes the following form:
$\frac{a}{b} = \frac{c}{d}$, where $a$ and $d$ are the extremes and $b$ and $c$ are the means.
In this exercise, $a = 12$, $b = m$, $c = 18$, and $d = 20$.
The extremes are $12$ and $20$. The means are $m$ and $18$.
Let's solve for $m$:
$\frac{12}{m} = \frac{18}{20}$
Use the cross products property to get rid of the fractions:
$18m = 240$
Divide each side by $18$ to solve for $m$:
$m = \frac{240}{18}$
Divide both the numerator and denominator by their greatest common factor, $6$:
$m = \frac{40}{3}$