Data Sufficiency

please explain this one

ketkoag wrote:please explain this one

DIFFICULT!

(1) a_2 = a_1 + k; a_1 = 24

Here we have no idea if k is positive or negative, which means we have no idea how many elements are greater than 10. If k was 1, all 15 elements would be greater than 10. If k was -24 we would have only 1 element greater than 10.

NOT SUFFICIENT.

(2) a_8 = 10 = a_7 + k

a_8 is smack in the middle of a_1 through a_15. If k were negative (say -1), then a_9 would 9, which is certainly be less than 10 and a_10 would be 8 and so on, giving us 7 elements greater than 10. The reverse applies if k were positive (say 1), then a_9 would be 11, a_10 would be 11 and so on, giving us 7 elements greater than 10. So irrespective of what the value of k is, we have 7 elements.

SUFFICIENT

(B)

nice explanation, thanks