abhi758 wrote:

If S is the sum of the first n positive integers, what is the value of n ?

(1) S < 20

(2) S^2 > 220

First n positive intergers: 1, 2, 3, 4, 5, 6, 7.

Sum of them could be: 3,6,10,15,21

(1) S < 20 -> n could be 1, 2, 3, 4, 5, -> insufficient

(2) S^2 > 220

220 = 11*2*10 = 11*4*5 -> S > square root of 220 = 2 * sqrt(55)

7^2 < 55 < 8^2 -> S > (about) 14

--> C ( S =15)

That is everything on my note in 1 min 20 sec.

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