It helped me to visualize the problem. I don't know if HN's formatting will kill this but here goes. The idea is that six customers arrive at 9:00, 01, 02, 03, 04, 05, and 06 - then map them out to "What happens when you add a new teller?" I tried to figure out how the author's explanation that adding one teller can go from 5 hours to 3 minutes.

Edit - the text version was a disaster here. Here is an image of what I used to try to understand:

http://i.imgur.com/HOKgfmd.png

One teller: average wait time is 22.5 minutes for "six users who arrive one minute apart from 9:00 to 9:05"

Two tellers: average wait time is 8 minutes.

I think to go from "5 hours with one teller" to "Three minutes with two tellers" requires you to space people out more than "one every one minute".