Answer:

problem

Answer	Crossword Clue
PROBLEM	Gordian Knot, e.g.
PROBLEM	Solver's target
PROBLEM	Textbook exercise

PROBLEM	"The Final ..." (Sherlock Holmes story)
PROBLEM	Math exercise
PROBLEM	Situation, so to speak
PROBLEM	Snag
PROBLEM	Hitch
PROBLEM	"No ...!" ("Piece of cake!")
PROBLEM	"Something wrong?"
PROBLEM	Sticky wicket

PROBLEM	Part of a math test
PROBLEM	Mr. Anthony's forte.
PROBLEM	Trifle for Univac.
PROBLEM	Riddle.
PROBLEM	Difficult matter.
PROBLEM	Perplexing situation
PROBLEM	Fly in the ointment, e.g.
PROBLEM	Gordian knot, for one

PROBLEM	Bugbear
PROBLEM	Bit of math homework
PROBLEM	Stumper
PROBLEM	"No ...!"
PROBLEM	Leak on a ship, e.g.
PROBLEM	Puzzler
PROBLEM	Mathematical challenge
PROBLEM	Challenge

PROBLEM	Math question
PROBLEM	Tough nut