Pattern of 20 tiles

This pattern of 20 tiles is arranged as 4 rows and 5 columns. The outer tiles around the edge are white and the inner tiles are black. When I change the number of rows and columns in my pattern I continue to keep the outer tiles around the edge white. 

a) Complete the following table:

b) I have 24 black tiles. What is the smallest number of white tiles I can use to make one of my patterns? Show workings to prove that your answer is the smallest possible solution.