The first five multiples of 3 are 3, 6, 9, 12 and 15.

Here, first term, a = 3, common difference, d = 6 – 3 = 3 and number of terms, n = 5

Using the formula,

Where S_n is the sum of first n terms

n = no of terms

a = first term

d = common difference

S₅ = (5/2)[ 2(3) + (5 - 1)(3)]

= (5/2)[ 6 + 12]

= 5(9)

= 45