154915

n=8 I know this because all of the numbers (7-15) add to 99 and there are 9 numbers so 99/9= 11, so the mean is 11. Then, because there are 3 numbers in each row, which each have the same total sum, you multiply 11 by 3 which equals 33. Now you know that each row/column/diagonal adds to 33. Since you know that, you can do 14+7=21 and 33-21=12 so the top right corner is 12. From there, you can use addition and subtraction to find the rest of the numbers (from left to right) ;the top row is 8,13,12 ;the middle row is 15,11,7 ;and the bottom row is 10,9,14.