A certain shop sells hats either retail or wholesale. For them, a wholesale is when the number of items purchased exceeds 20 pieces. For retail, each hat sells for Php 100.00. However, for wholesale transactions, those hats in excess of 20 pieces will now be sold as Php 95.00 each. Let n be the number of hats purchased and suppose a(n) represents the total amount upon purchase of n hats. Represent a(n) as a piecewise function.