Is it possible to make a rule "a = a+1"?

[ioscat]

Hello.
Is it possible to make a rule "a = a+1"?
Now I'm going through IBM Cognos training and while making one of the tasks I got the question. The original task is something like:
b = a;
c = count (a);
a = b/c
So, why can't we just write a = a / c ?

Additional information:
"a" have hierarchy with simple and consolidate elements.