#### Answer

We get the value of $C\left( 100 \right)=\$36$.

#### Work Step by Step

$C\left( t \right)=20+0.4\left( t-60 \right)$
Where $C\left( t \right)$ is the monthly cost in dollars and $t$ is calling minutes.
For $t=100$:
$\begin{align}
& C\left( 100 \right)=20+0.4\left( 100-60 \right) \\
& =20+0.4\left( 40 \right) \\
& =20+16 \\
& =36
\end{align}$
Thus, for 100 calling minutes, the monthly cost is $36.