You want the default permissions for your files to bE.rw-r-. How must you set umask?

To achieve `rw-r-----` (octal 640) for files, the `umask` must remove no permissions for the owner (0), remove write and execute for the group (3), and remove all permissions (read, write, execute) for others (7). Applying `umask 037` to the default file permissions of `rw-rw-rw-` (666) results in `rw-r-----` (640).

`640` represents the desired file permissions, not the `umask` value used to achieve them.

`038` is an invalid octal `umask` value because '8' is not a valid octal digit.

While `umask 027` would also result in `rw-r-----` (640) for files, it specifies removing only the write permission for the group (2) as opposed to write and execute (3) provided by `037`.