comp.os.linux.networking

I am taking a course at a local university and we're using a revision
of "Red Hat Linux Networking and System Administration." In the VNC
configuration section, the text states:

"By default, VNC uses ports numbered 5900 plus the display number. In
this example, the display is :1, so the port number is 5901. If you
were using display number :21, the port number would be 5912."

Is the port designation that is transposed for display number :21
supposed to be transposed to "5912" as stated? Or is this a typo and
should it be "5921?"

TIA,
Dave