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Retake_GMAT · 07-23-2008, 06:13 AM

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

07-23-2008, 06:13 AM	#2 (permalink)
Retake_GMAT Chill Join Date: May 2008 Location: India, Bangalore Posts: 862	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(22+(n-1)2)=n(n+1) and this is equal to 79*80 therefore n=79 which is odd