"Why are waiters good at math?"/"They know…

A riddle about knowing multiplication tables is:

Q: Why are waiters good at math?
A: They know all their tables.

The joke was included in a 1983 book on riddles and puns.

Google Books
Remember the a la mode:
Riddles and Puns
By Charles Keller
Illustrated by Lee Lorenz
Englewood Cliffs, NJ: Prentice-Hall
1983
Pg. ?:
Why are waiters good at sums?
Because they know their tables.

Google Groups: alt.humor
Puns of the Weak 02/01/02
Stan Kegel
2/1/02
(...)
Waiters are good at multiplication because they know their tables. (Pun of the Day)

3 February 2002, Sunday Herald-Sun (Melbourne, Australia), “Laugh out loud,” pg. F6:
Q.Why are waiters so good at sums?
A.Because they know their tables.
Liam, 8, Rosebud

Google Groups: alt.humor.puns
Puns of the Weak 04-25-03
Stan Kegel
4/25/03
(...)
Why are waiters good in multiplying?
Because they know their tables (Jennie, 11)

Twitter
fabulous savings
‏@FabulousSavings
Why are waiters so good at multiplication? They know their tables! Get 80% off at @Restaurant_com w/ our free #coupons: http://ow.ly/67P63
10:30 PM - 20 Aug 2011

Facebook
Bronx Science Joke of the Day
May 4, 2015 ·
Joke #99:
“Why are waiters good at math?”
They know all their tables!

Twitter
Shanz Dev
‏@ShanzDev
Why are waiters good at multiplication?
Because they know their tables!
Ha! See how that went?! Yeah!
#ShanzHumor
Happy Tuesday!!
8:18 AM - 26 Jan 2016

“Why are waiters good at math?”/“They know all their tables.”