subject is algebraic systems

Problem 4. Determine whether the given operation is a binary operation on Z. If so, state (i) whether it is commutative or not, (ii) whether there is an identity element for the operation and (iii) whether the set of odd integers is closed with respect to the operation? Justify your answers! (1) a+b=a - 26. (2) a*b=a. (3) a+b=a/b. (4) a*b = a + b - ab