161. How many six-digit natural numbers exist, each of which has its digits arranged in ascending order?
$\triangle$ Let's take the nine-digit number 123456789, whose digits are in ascending order. To obtain any six-digit number with the same property, it is sufficient to erase any three digits from this nine-digit number. Since the order of the digits being erased does not matter, we are dealing with combinations. We have:

$$
C_{9}^{3}=\frac{9 \cdot 8 \cdot 7}{1 \cdot 2 \cdot 3}=84
$$

Answer: 84.