In a school,students thought of planting tress in and around the school to reduce air pollution.It was decided that the number of trees that each section of each class will plant will be the same as the class in which they are studing e.g.a section of I class will plant 1 tree,a section of II class will plant 2 trees and so on,a section of XII class will plant 12 trees.There are three sections of each class. Find out how many trees were planted.