Question

Find the sum of odd numbers which are divisible by 3 and lie between 1 and 500

Answer 1

20,667.

Explanation:

This is an arithmetic series:

3 + 9 + 15 + 21 + 27+ ...........+ 495

First term = 3 and common difference = 6.

Number of terms = ( 495 - 3 )/6 + 1

= 83.

So Sn = n/2(a1 + l) where a1 = first term and l = last term

S83 = 83/2(3 + 495)

= 41.5 * 498

= 20,667.