First 15 multiples of 8 are 8, 16, 24, 32,…………..

This, clearly is an AP

where,

a = 8

And,

d = 16 – 8 = 8

We know that,

Sum of first n terms of an AP,

⇒ S₁₅ = 15 × 64

⇒ S₁₅ = 960

Hence, sum of first 15 multiples of 8 = 960