Ubuntu 14.04: User is in a group but cannot edit files?

Question

linux

Ubuntu 14.04: The user dima154 (i.e. me) is in the www-data group but cannot edit files in /var/www/... etc., which is owned by the www-data user. When changing the owner for /var/www/ to dima154, everything starts working, in the opposite direction - no.

Answer 1

Well apparently you can edit test.php for example, but count.txt can't.
Because even in your calculations it is clear that the group does not have the rights to edit this file.

Answer 2

group and owner are different things
Can one user edit files for another?

Answer 3

means the group is not given editing permissions

Answer 4

But as I understand it, if I am a member of a group that owns a directory, then I can edit the contents of this directory?