20. Ans: 102

Note that from $a_{n}=10 a_{n-1}-1$, we have
$$
a_{n}-\frac{1}{9}=10\left(a_{n-1}-\frac{1}{9}\right)
$$
for all $n \geq 2$. Thus,
$$
a_{n}-\frac{1}{9}=10^{n-1}\left(a_{1}-\frac{1}{9}\right)=10^{n-1} \frac{8}{9}
$$
for all $n \geq 1$. Therefore
$$
a_{n}=\frac{\left(1+8 \times 10^{n-1}\right)}{9} .
$$

Observe that for $n \geq 2$,
$$
8 \times 10^{n-2}<a_{n}<10^{n-1} .
$$

Thus
$$
a_{101}<10^{100}<a_{102} .
$$

That is why the answer is 102 .
20. Let $a_{1}, a_{2}, \ldots$ be a sequence satisfying the condition that $a_{1}=1$ and $a_{n}=10 a_{n-1}-1$ for all $n \geq 2$. Find the minimum $n$ such that $a_{n}>10^{100}$.