"There's a potential ambiguity- the singular could suggest they were working together and the plural that they had more than one definition, so I would rewrite it and say that they were unable to finish the task."
Consider this context:
Whenever I ask my students whether they can define ‘metaphysics’, they most often produce answers like
"Metaphysics is a concept with fuzzy boundaries, if any. Whenever I ask my secondary school students (14 to 18 year olds) whether they can define it, they most often produce answers like “Of course, I can. Metaphysics is … hm, you know …” (and this is the moment when they realize that they will not be able to finish their definition) or “Sure. Metaphysics is
"Metaphysics is a concept with fuzzy boundaries, if any. Whenever I ask my secondary school students (14 to 18 year olds) whether they can define it, they most often produce answers like “Of course, I can. Metaphysics is … hm, you know …” (and this is the moment when they realize that they will not be able to f
How would you rewrite this paragraph to look more formal?
Can you give it a try? In many cases, including yours here, parentheses do not make the meaning very clear. I don't want to start trying to guess your precise meaning? As I say, try to rewrite it yourself.