Question

Find the sum of the first 15 multiples of 8.

• A. 1000
• B. 960
• C. 500
• D. 820

Answer 1

The correct option is B

960

The first 15 multiples of 8 are 8, 16, 24, 32, .... 112, 120

First term, $a=8$

Common difference, $d=16-8=8$

$n=15$

Applying formula, $S_n=\frac{n}{2}(2a+(n-1\left)d\right)$ to find sum of $n$ terms of AP, we get
$S_{15}=\frac{15}{2}\left(2\right(8)+(15-1\left)\right(8\left)\right)$
$=\frac{15}{2}(16+8(14\left)\right)$
$=\frac{15}{2}(16+112)$
$=\frac{15}{2}\left(128\right)=960$