Sometimes it is none of those.
Here are a few clues that use numbers as mathematical objects and symbols to stand for arithmetic operations.
Can you solve them?
From Alec Robins' book The A.B.C. Of Crosswords:
|⅔ of 1 + ⅔ of 1 – what kind of sign is that? (4)|
From Anatomy of the Crossword by D. St. P. Barnard:
Guardian 25037 (Puck):
|Poor fish eaten by roughly 4.10 (11)|
|(twelfth of seventy-eight) + (fourth of five) + (eighth of eighteen) (3)|
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