The first 15 multiples of 8 are 8,16,24,32,40,48,56,64,72,80,88,96,104,112,120.  As the difference between two consecutive multiple is 8, so they are forming a. Arithmetic progression.

Here a=8.

d= 8

Sum of first 15 multiples is given by Sn = n/2[ 2a +( n-1)d]

Putting value of a, d and n

Sn = 15/2[ 2×8+ (15-1)8]

Sn = 960