b10m has a good solution based on a common Unix tool (that's available for other platforms, but not as common on them), but how about a Perl solution?

Here's a one-liner you can use from the command line:

perl -ne 'next if 0 == $. % 25; print' filename
[download]

Here's a somewhat clearer version:

#!/usr/local/bin/perl --
use strict;
use warnings;

my $remove_lines_modulo = 25;

while ( <> ) {
    next if 0 == $. % $remove_lines_modulo;
    print;
}
[download]

Or one that accepts an argument for which lines to remove:

#!/usr/local/bin/perl --
use strict;
use warnings;

my $remove_lines_modulo = shift @ARGV;
die "Cannot handle zero as an argument.\n" if 0 == $remove_lines_modul
+o;

while ( <> ) {
    next if 0 == $. % $remove_lines_modulo;
    print;
}
[download]

Please remember that if this is a homework assignment, it's proper form to say so. Asking for completed work instead of help when it's a school assignment doesn't do any good in the long run and isn't fair to students who actually did the work.