17. Given a set of $n$ positive integers, the difference between any two elements is either divisible by 5 or by 25, then the maximum value of $n$ is $\qquad$ .
Consider a 5-digit number, first fix the unit digit, for example, 1. Construct 25 numbers as follows: $1,11,21,31,41 ; 101,111,121,131,141$; 201, 211, 221, 231, 241; 301,311,321,331,341;401,411,421,431, 441.
For other 5-digit positive integers, if their unit digit is not 1, then the difference between them and each of the above numbers is definitely not a multiple of 5; if their unit digit is also 1, then the difference between them and at least one of the above numbers has the last three digits as 0, thus the difference is a multiple of 125. Therefore, the maximum value of $n$ is 25.