The first 15 multiples of 8 are 8, 16, 24, …, 120.
Here, a = 8,d = 16 – 8 = 8 and n = 15.
The sum of n terms of an AP is gevin by
S_n = n/2[2a + (n -1)d]
⇒ S₁₅ = 15/2[2(8) + (15 -1)(8)]
= 15/2[16 + 112]
= 15/2(128)
= 960
Hence, the sum of the 15 multiples of is 960.