Originally Posted by

**KayPee** I have been struggling to understand the question and answer below I came across in my GMAT class.

Can anyone explain the Q and A below and set another question very similar to this question for me to try and resolve?

**Q1.The sum of the even numbers between 1 and n is 79*80, where n is an odd number, then n=?**

**A.** Sol: First term a=2, common difference d=2 since even number

therefore sum to first n numbers of Arithmetic progression would be

n/2(2a+(n-1)d)

= n/2(2*2+(n-1)*2)=n(n+1) and this is equal to 79*80

therefore n=79 which is odd