Question

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 same as the class, in which they are studying, e.g., a section of class $\text{I}$ will plant $1$ tree, a section of class $\text{II}$ will plant $2$ trees and so on till class $\text{XII}$, If there are three sections of each class, then the number of trees planted is

• A. $78$
• B. $234$
• C. $39$
• D. $156$

Answer 1

The correct option is B $234$

The number of tree planted by class $\text{I}$ is $1\times 3.$
The number of tree planted by class $\text{II}$ is $2\times 3.$
The number of tree planted by class $\text{III}$ is $3\times 3.$
$⋮$
The number of tree planted by class $\text{XII}$ is $12\times 3.$

The total number of tree planted
$=3+6+9+\dots +36$
Using sum of $n$ terms formula of A.P.
$=\frac{12}{2}[3+36]=234$