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4.Average
easy
The average of the first nine integral multiples of $3$ is
A
$12$
B
$15$
C
$18$
D
$21$
Solution
Sum of the $9$ integral multiple of $3 .$
$=\frac{9}{2}[2 \times 3+(9-1) 3] \quad\left[\begin{array}{l}\text { Using Formula } \\ S_{n}=\frac{n}{2}[2 a+(n-1) d]\end{array}\right]=135$
Average $=\frac{135}{9}=15$
Standard 13
Quantitative Aptitude