I3.3 Let $f(x)=\left\{\begin{array}{ll}x+5 & \text { if } x \text { is an odd integer } \\ \frac{x}{2} & \text { if } x \text { is an even integer }\end{array}\right.$.
If $c$ is an odd integer and $f(f(f(c)))=b$, find the least value of $c$.
$f(c)=c+5$, which is even
$$
f(f(c))=\frac{c+5}{2}
$$

If $\frac{c+5}{2}$ is odd, $f(f(f(c)))=18$
$$
\begin{array}{l}
\Rightarrow \frac{c+5}{2}+5=18 \\
\Rightarrow c+5=26 \\
\Rightarrow c=21
\end{array}
$$

If $\frac{c+5}{2}$ is even, $f(f(f(c)))=18$
$$
\begin{array}{l}
\Rightarrow \frac{c+5}{4}=18 \\
\Rightarrow c+5=72 \\
\Rightarrow c=67
\end{array}
$$

The least value of $c=21$.