Anybody read An Approach to general systems theory by Klir ?

There is a question in chapter 1 that has buggered me silly.
Here it is:
How many structural resolution levels exist in a system
a)Containing 3 elements whose behaviors aren't decomposible?

The answer is 5(number of partitions on a1,a2,a3) .

I read the chapter and can't figure the answer to it.
HELP !