**K. 198.** The inhabitants of a certain planet use not four but five basic arithmetic operations. The operations of addition, multiplication, subtraction and division are the same as ours, but they also have a special operation denoted by the operation sign @. We do not know exactly how this operation works, but we have found out that the following properties are valid for all *x* and *y*:

(*a*) ,

(*b*) ,

(*c*) .

What is the value of on the planet in question?

(6 points)

This problem is for grade 9 students only.

**Deadline expired on 10 February 2009.**