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Entry from July 14, 2020
“5Q + 5Q = ?"/ “10Q."/ “You’re welcome.”

A old math joke is:

“5Q + 5Q = ?”
“10Q.”
“You’re welcome.”

“10Q” sounds like “thank you.” The joke has been cited in print since at least the 1920s.

The Cornell Civil Engineer
Volumes 37-38
1928
Pg. 196:
HIGHER MATHEMATICS
C.E. ‘31— “By the way, you think you know your calculus. Then what is 5Q plus 5Q?”
C.E. ‘32— “10Q, of course.”
C.E. ‘31— “You’re welcome.”

Newspapers.com
25 September 1933, Birmingham (AL) News, “The Coal Bin” by Henry Vance, pg. 8, col. 6:

January 1937, Boys’ Life, “Think and Grin” edited by Frank J. Rigney, pg. 23, col. 1:
4U
Soph: How much is 5q plus 5q?
Fresh: Ten q.
Soph: You’re welcome.

July 1973, Boys’ Life, “Think and Grin,” pg. 66, col. 2:
Willy: What’s 5Q plus 5Q?
Nilly: Uh— 10Q.
Willy: You’re welcome! — John Garcia, South El Monte, Calif.

Broken Window
Bob N Keenan
8/8/90
(...)
~ Bob N. Keenan | ALGEBRA QUIZ
~ University of Wisconsin-Milwaukee | Q: What is 5q+5q?
~ | A: You’re welcome!

Brian Myers
12/29/97
(...)
13.  Please leave your name and number—But first, a short algebra quiz: How much is 5Q + 5Q? (Pause while caller thinks: 10Q) You’re welcome!

alyssa lovato
@alyssahts
What’s 5Q+5Q?......10Q....."You’re welcome” Hahahaha
1:21 PM · Jun 14, 2009·Twitter Web Client

Jordan
@jordankhorst
What’s 5Q+5Q?
...you’re welcome.
2:49 PM · Aug 10, 2014·Twitter for Android

Reddit—Jokes
Posted by u/ThreadlessJon August 15, 2015
A math teacher ask his students, “What is 5Q + 5Q?”
The class responds, “10Q.”
The teacher responds, “You’re welcome/”
J_Gizzy
Whats 2Q + 2Q?

Posted by u/justinsjams April 28, 2019
Whats 5Q + 5Q?
“10Q”
You’re welcome.

Tom Osborn, PhD, FRSA.
@call_me_tomasso
Dad/Maths joke. It’s been a while, so here…
5Q + 5Q = ?
You’re welcome.
11:39 PM · Jul 13, 2020·Twitter Web App