Question

The sum of first five multiples of 3 is
(a) 45
(b) 55
(c) 65
(d) 75

Answer 1

First five multiples of 3 are:
3, 6, 9, 12, 15

They are in A.P. with
First term = a = 3
Common difference = d = 6 − 3 = 3

$$\begin{gathered} S_5=\frac{5}{2}\left[2a+\left(5-1\right)d\right] \\ =\frac{5}{2}\left[2a+4d\right] \\ =\frac{5}{2}\left[2\left(3\right)+4\left(3\right)\right] \\ =\frac{5}{2}\left[6+12\right] \\ =\frac{5}{2}\times 18 \\ =45 \end{gathered}$$

​Hence, the correct option is (a).