Totally useless but a little bit of fun with a silly mathematical property:

#!/usr/bin/env perl                                                   
+  
                                                                      
+  
use strict;                                                           
+  
use warnings;                                                         
+  
use List::Util qw(sum);                                               
+  
                                                                      
+  
# For each $n, convert $n to hex digits, add the digits together. If t
+he
# result has more than one hex digit, repeat this process adding the  
+  
# digits together. When there is only one hex digit remaining, if the 
+  
# digit is a '5', an 'a', or an 'f', the original number was divisible
+  
# by 5.                                                               
+  
                                                                      
+  
for my $n (1..100) {                                                  
+  
    my $h = sprintf '%x', $n;                                         
+  
    while(length($h) > 1) {                                           
+  
        $h = sprintf '%x', sum(map {hex $_} split //, $h);            
    }                                                                 
+  
    print "$n is a multiple of 5\n" if $h =~ m/^[5af]$/;              
+   
}
[download]

Output:

5 is a multiple of 5
10 is a multiple of 5
15 is a multiple of 5
20 is a multiple of 5
25 is a multiple of 5
30 is a multiple of 5
35 is a multiple of 5
40 is a multiple of 5
45 is a multiple of 5
50 is a multiple of 5
55 is a multiple of 5
60 is a multiple of 5
65 is a multiple of 5
70 is a multiple of 5
75 is a multiple of 5
80 is a multiple of 5
85 is a multiple of 5
90 is a multiple of 5
95 is a multiple of 5
100 is a multiple of 5
[download]