The above is a GMAT Prep test data sufficiency question. Which I'd like to understand how to solve, as I didn't even know really where to start.

My only thoughts were..

St1) avg = 7 = x/9, x (sum of first 9) = 63
St2) avg = 9 = x/9, x (sum of last 9) = 81

I realize the middle 7 numbers excluding the first 2 & last 2 are shared. But I don't know if that gets me anywhere.

Please help. Thanks

Option 1:

as 7 is the mean it is the middle no (put 4 consecutive numbers on either side of 7) so the first 9 numbers are:
3 4 5 6 7 8 9 10 11

hence for average of 11 numbers we include 12 and 13 as numbers are consecutive

= (3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13)/11


Option 2:

as 9 is the mean it is the middle no (put 4 consecutive numbers on either side of 9) so the last 9 numbers are:
5 6 7 8 9 10 11 12 13

hence for average of 11 numbers we include 3 and 4 as numbers are consecutive

= (3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13)/11

Cheers!