1. Arithmetic Progression

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

2. Re: Arithmetic Progression

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
What don't you understand in the above solution? And doesn't your textbook have similar questions?