Prove by induction that the number of 2-subsets of an n-set A equals n(n-1)/2.
Base Step: When , it is trivial (we have 2-subset); when , we have 2-subset.
Inductive Step: Now lets assume when , we have 2-subsets. Consider the case when (i.e. we add one more element into the set, so how many more 2-subsets will be introduced? The answer is more 2-subsets are introduced. Think about this)
So the total number of 2-subsets when is , this competes the inductive step.