i know session variables store on the server side and cookies store on client side.

Question: If i save the total answer for a summation as a session variables, and multiple online user are using the same system for calculation, does that means that session variable will be over write by other value where other user have different summation answer?

Do you mean if one user's action will change the other's session value?

Server will creates a new Session for each new user, and destroys the Session when it is expires. User cannot modify the other users' seesion value.