Sorry, great programmers, for stupid questions in this topic, but I'm study and I suggest to make this topic for beginners.
PLC V350, example, math function A+B=C . I set block ADD in Ladder, in HMI set 3 numeric button for A,B,C. I made connections in variables to A,B,C, I enter numbers in 1st and 2nd numeric button, but in 3rd (C) not changes (addition).