3. Entropy is also defined when classification is into three or more classes.

The entropy of a set S relative to n distinct classes is defined as:
Entropy (S) = 2 1 log n i i i p p = Σ−
where pi is the proportion of the Set S in class i, with i = 1, …, n. Notice that logarithms are still calculated base 2.
Calculate the entropy of the set S, where p1 = 6/20, p2 = 9/20, and p3 =5/20.