3. A five-digit number is called humped if its middle digit is greater than the others, and valley-like if this digit is smaller than the others. Which are more numerous - humped or valley-like numbers?
3. Answer: ravine.

Solution: There is a one-to-one correspondence between ravine numbers not starting with 9 and all humpback numbers, given by the formula: $x \leftrightarrow 99999-x$.

Ravine numbers starting with 9 are extra.

## Instructions for checking:

Students can simply count the number of both types of numbers.

Watch for the absence of errors.