I choose E.
1. n(n+1)/2 = 3124 => solving for n gives -3 or 2. Therefore 2. Sufficient
2. n(n+1)/2n = 4 => solving for n gives 0 or 7. Therefore 7. Sufficient
Not really sure where you're going here. First, if you think each statement is sufficient, you should be choosing D, not E. You also seem to be assuming that the sequence is a sequence of consecutive integers beginning from 1 - that's the only situation where you could use the "sum = n(n+1)/2" formula. You aren't told what kind of sequence this is, so you can't use any formula for the sum here. Finally, I don't understand how you arrived at the solutions -3 and 2 from the equation n(n+1)/2 = 3124.
prosper's solution is correct.
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