This is the basic question from arithmetic progression chapter in which we are to calculate the sum of the first 15 multiples of 8.
Book – RS Aggarwal, Class 10, chapter 5C, question no 15
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The multiples of 8 are 8,16,24,32…
These are in an A.P., having first term as 8 and common difference as 8.
Therefore, a=8,d=8, S15=?
Sn=n/2[2a+(n−1)d]
S15=15/2[2(8)+(15−1)8]
=15/2[16+(14)(8)]
=15/2×128
=960