greenwich,

u could check the following links.

https://www.beatthegmat.com/data-suffici ... 47978.html

https://gmatclub.com/forum/the-sequence- ... 99318.html.

It is always a good idea to google the question Most often than not, you are bound to find an explanation
Hope this helps.

strongly agree with aimscore. however, still posting questions in the forum helps all a lot and the discussion makes the idea much clear. So keep posting and googling

Sum = ( First term + last term /2) * n { formula to find sum when the enitities are in a sequence)

= {1 + 1/ n ( n+1)/ 2 } * n

= n ( n+1) +1 / 2 * ( n+1) --- eqn 1



St 1 : K > 10

So minimum K = 11

Put K = 11 in equation 1 :

(11 * 12 +1) /2 * ( 11+1)

133/24

133/24 is > 9/10

Sufficient

St 2:

K < 19

let K = 1 ; Sum = n ( n+1) +1 / 2 * ( n+1)
= 3/4 = 0.75

We have sum < 9/10

Let K = 18 ;Sum = 18 * 19 +1 / ( 2 * ( 18+1)
= 343 / 38
sum is > than 9/10
We have consistent values.
Insufficient

Pick A