Question

The sum of first 15 multiples of 8 will be:

• A. 960
• B. 980
• C. 984
• D. 990

Hint:

For sums of multiples, always form an arithmetic sequence and use \( S_n = \frac{n}{2} [2a + (n-1)d] \).

Answer 1

The Correct Option is A

Step 1: Identify the sequence.
The multiples of 8 are: 8, 16, 24, 32, ... This forms an arithmetic progression (A.P) with first term \( a = 8 \) and common difference \( d = 8 \).

Step 2: Use the sum of first \( n \) terms of A.P.
\[ S_n = \frac{n}{2} [2a + (n-1)d] \]
Step 3: Substitute values.
\[ S_{15} = \frac{15}{2} [2(8) + (15-1)(8)] = \frac{15}{2} [16 + 112] = \frac{15}{2} \times 128 = 960 \]
Step 4: Final answer.
\[ \text{Sum} = 960 \]