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First Number: a = 2
Difference: d = 2
nth number: a+(n-1)d = 2+(n-1)2 = 2+ 2n-2 = 2n

Sum 79*80 = (1/2) n (first +nth number) = (1/2) n (2+2n) = n(n+1)

n=79

by alpha_gmat

Mon Aug 28, 2006 10:46 am
Forum: Problem Solving
Topic: Difficult Math Question #1
Replies: 30
Views: 24733