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