The Pentagon is the headquarters of the United States Department of Defense and is located in Arlington County, Virginia. About 30,000 people work in the Pentagon. An old joke is told about a legislator who entered and asked an elevator operator, “How many people work in the Pentagon?” The elevator operator replied, “Oh, about half.” The Pentagon joke dates to at least 1957.
 
The Pentagon opened in 1943, but the work joke is older than the building. “Q. How many people work here? A. Oh, about half” is a joke that dates to at least 1918.
 
   
Wikipedia: The Pentagon
The Pentagon is the headquarters of the United States Department of Defense, located in Arlington County, Virginia. As a symbol of the U.S. military, “the Pentagon” is often used metonymically to refer to the U.S. Department of Defense.
 
The Pentagon was designed by American architect George Bergstrom (1876–1955), and built by general contractor John McShain of Philadelphia. Ground was broken for construction on September 11, 1941, and the building was dedicated on January 15, 1943. General Brehon Somervell provided the major motive power behind the project; Colonel Leslie Groves was responsible for overseeing the project for the U.S. Army.
 
The Pentagon is a large office building, with about 6,500,000 sq ft (600,000 m2), of which 3,700,000 sq ft (340,000 m2) are used as offices. Approximately 28,000 military and civilian employees and about 3,000 non-defense support personnel work in the Pentagon. It has five sides, five floors above ground, two basement levels, and five ring corridors per floor with a total of 17.5 mi (28.2 km) of corridors. The Pentagon includes a five-acre (20,000 m2) central plaza, which is shaped like a pentagon and informally known as “ground zero,” a nickname originating during the Cold War on the presumption that it would be targeted by the Soviet Union at the outbreak of nuclear war.
 
31 December 1957, Boston (MA) Herald, “The Lyons Den” by Leonard Lyons, pg. 21, cols. 4-5:
WORK: John C. Stennis (D., Miss.), as a member of the subcommittee investigating our missles program, went to the Pentagon recently. He was greeted by a Pentagon elevator operator who is one of his constituents. On the way upstairs Se. Stennis asked: “How many people work in the Pentagon?”...The elevator man replied: “Oh, about half.”
   
Google Books
MSU Alumni Magazine
Volumes 1-3
1983
Pg. 8:
How many people work in the Pentagon? “About half,” he allows.
 
Google Books
The Economics of Chaos:
On Revitalizing the American Economy
By Eliot Janeway
New York, NY: Dutton
1990, ©1989
Pg. 246:
The resultant debacle recalled a more than normally cynical response of Forrestal’s to the question “How many people work at the Pentagon?” “About half,” he snapped.
 
Google Groups: rec.arts.sf.tv
Another B5/Straczynski project rumored
Arthur Levesque
8/1/00
(...)
Probably plays Solitaire all day.  That seemed to be the main job that got done by people when I worked in the Pentagon; thus the joke:
Q:  How many people work in the Pentagon?
A:  At any given time, about 10% of them.
     
InvestorVillage
Posted 10/9/2008 3:47:48 PM by Plute
We Need To Reduce Size of Government and Obama Will Not.
There was an old joke when asked “How many people work at The Pentagon?” Answer: About half. Actually there are more freeloaders in all the other parts of Government. Pay in DC is about $90.000/employee as this is not Gurnee or Tulsa. All the guys that worked for me now get $149,000/year + bonuses. Ever try to fire one? I did and gave up after a year!
 
Twitter
ClearanceJobs ‏@ClearanceJobs Jul 15
Who says the Pentagon doesn’t have a sense of humor? Just heard this old joke: ‘How many people work in the Pentagon?’ Who knows the answer?
 
Nick Martin
‏@TriSpaceMtn
@ClearanceJobs On a good day? About half of them!
8:40 PM - 15 Jul 2014