“Why was the math book sad?”/“Because it had so many problems.”

A math riddle has been printed on several images:
 
Q: Why was the math book sad?
A: Because it had so many problems.
   
The riddle was printed in many newspapers in 1987.
 
“I’m not sure how many problems I have because math is one of them” is another joke about math problems. 
 
   
3 July 1987, Indiana (PA) Gazette “Mighty Funny’s Mini Jokes,” The Mini Page, pg. 3, col. 2:
Why was the math book sad?
It had too many problems!
 
4 September 1988, The Arizona Republic (Phoenix, AZ), “State youths share favorite jokes, riddles,” Kids page?, col. 2:
Question: Why was the math book so unhappy?
Answer: It had so many problems.
Melody Rector, 10 Phoenix
   
17 June 1990, Newsday (Long Island, NY), “Smiles,” Kidsday, pg. ?”
Why was the math book crying? It had too many problems. - Fiona McGinn, Sound Beach
   
17 April 1994, Washington (DC) Post “Mighty Funny’s Mini Jokes,” The Mini Page, pg. 3, col. 2:
Why was the math book sad?
It had too many problems!
 
Google Books
Squeaky Clean Jokes for Kids
By Bob Phillips and Steve Russo
Eugene, OR: Harvest House Publishers
1997
Pg. 52:
Tony: Why is a math book always cranky?
Tobiah: I have no idea.
Tony: Because it has lots of problems.
 
23 June 2000, Los Angeles (CA) Times, “The Kids’ Reading Room; Jokes & Riddles, pg. E8, col. 5:
Why is a math book always sad? It has too many problems. (Jasmine, 10, Castlebay Lane Elementary, Northridge)
   
Google Books
A Prairie Home Companion
Pretty Good Joke Book
By Brian Becker
Minneapolis, MN: HighBridge
2003
Pg. 60:
Why was the math book sad?
Because it had so many problems.
   
Twitter
Miguel Lopez
‏@mklopez
“Why is a math book so sad?” “Because it has many problems”
7:40 PM - 20 Oct 2008
 
Google Books
Belly Laugh Jokes for Kids:
350 Hilarious Jokes
By Sky Pony Editors
New York, NY: Skyhorse Publishing
2015
Pg. ?:
Q: Why was the math book sad?
A: Because it had too many problems.
   
Reddit—Jokes
Why was the math book sad?
submitted February 2, 2017 by Pm_Me_Jill_Valentine
It had lots of problems.
[My little brother told me this earlier.]