In a school, students thought of planting trees 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 studying, e.g., a section of Class I will plant 1 tree, a section of Class II will plant 2 trees and so on till Class XII. There are three sections of each class. How many trees will be planted by the students?

First there are 12 classes and each class has 3 sections

Since each section of class 1 will plant 1 tree, so 3 trees will be planted by 3 sections of class 1.

Thus every class will plant 3 times the number of their class 
(for example class (iii) will plant = 3 x 3 = 9 plants) 

Similarly,

No. of trees planted by 3 sections of class 1 = 3

No. of trees planted by 3 sections of class 2 = 6

No. of trees planted by 3 sections of class 3 = 9

No. of trees planted by 3 sections of class 4 = 12

Its clearly an AP with first term = Number of trees planted by class 1 = 3

We have; a = 3, d = 3 and n = 12

We can find the total number of trees as follows:

= 6 (6 + 33)

= 6 x 39

= 234

Total number of trees planted by students = 234