Answer
The caller can talk for more than 80 minutes but less than 110 minutes.
Work Step by Step
The desired range of calls can be obtained by solving the following two inequalities:
\[\begin{align}
& C\le 40 \\
& C\ge 28 \\
\end{align}\]
The first inequality can be solved as follows:
\[\begin{align}
& 28\le 20+0.40(x-60) \\
& 28-20\le 20+0.40(x-60)-20 \\
& 8\le 0.40(x-60) \\
& \frac{8}{0.40}\le \frac{0.40(x-60)}{0.40}
\end{align}\]
Simplify further to obtain,
\[\begin{align}
& 20\le x-60 \\
& 20+60\le x-60+60 \\
& 80\le x
\end{align}\]
Hence, the caller can talk for more than 80 minutes.
Now solve the second inequality as follows:
\[\begin{align}
& 20+0.40(x-60)\le 40 \\
& 20+0.40(x-60)-20\le 40-20 \\
& 0.40(x-60)\le 20 \\
& \frac{0.40(x-60)}{0.40}\le \frac{20}{0.40}
\end{align}\]
This can be further simplified as:
\[\begin{align}
& x-60\le 50 \\
& x-60+60\le 50+60 \\
& x\le 110
\end{align}\]
Hence, the caller can talk for less than 110 minutes.
The caller can talk for more than 80 minutes but less than 110 minutes.
