question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

(arithmetic) arithmetic operator

unary operator (operator taking one operand)

(mathematics, computing) operand

(mathematics) A procedure for generating a value from one or more other values; operation.

to calculate

use one thing for many purposes / work someone hard

to use one's head

(algebra, computing) binary operator

necessity (quality or state of being necessary, unavoidable, or absolutely requisite)