29. A monkey is climbing up and down a ladder with a total of $n$ steps. Each time it ascends 16 steps or descends 9 steps. If it can climb from the ground to the very top step and then return to the ground, what is the minimum value of $n$?
29. $n=24$, the monkey can climb to the top and return to the ground,
$$
\begin{array}{l}
0 \rightarrow 16 \rightarrow 7 \rightarrow 23 \rightarrow 14 \rightarrow 5 \rightarrow 21 \rightarrow 12 \rightarrow 3 \rightarrow 19 \rightarrow 10 \rightarrow 1 \rightarrow 17 \rightarrow 8 \rightarrow 24, \\
24 \rightarrow 15 \rightarrow 6 \rightarrow 22 \rightarrow 13 \rightarrow 4 \rightarrow 20 \rightarrow 11 \rightarrow 2 \rightarrow 18 \rightarrow 9 \rightarrow 0 .
\end{array}
$$
$n=23$, can climb to the top but cannot return to the ground,
$$
\begin{array}{l}
0 \rightarrow 16 \rightarrow 7 \rightarrow 23, \\
23 \rightarrow 14 \rightarrow 5 \rightarrow 21 \rightarrow 12 \rightarrow 3 \rightarrow 19 \rightarrow 10 \rightarrow 1 \rightarrow 17 \rightarrow 8,8+
\end{array}
$$
$$
16>23,8-9n, 7-9<0$, cannot climb.