2. A five-digit number is called humped if its first three digits are in strictly increasing order, and its last three digits are in strictly decreasing order, and it is called ravine if its first three digits are in strictly decreasing order, and its last three digits are in strictly increasing order. Which numbers starting with 5 are more - humped or ravine?
2. Answer: ravine.

Solution: We will only consider numbers starting with 5. Between the ravine numbers, the second digit of which is different from 4, and all humpback numbers, there is a one-to-one correspondence according to the formula: $x \leftrightarrow 109999$ - x. Ravine numbers starting with 54 are extra.

Instructions for checking: students can simply count the number of both types of numbers; watch for no errors.