How many ways can you distribute $4$ different balls among $4$ different boxes?
How many ways can you distribute $4$ identical balls among $4$ identical boxes?
How many ways can you distribute $4$ identical balls among $4$ different boxes?
(a) There are 4*4 = 16 ways.
(b) There are six ways.
(c) There are 16 + 14 = 30 ways.
+51 
