A5. In a magic square, the sum of the three numbers in each row, the sum of the three numbers in each column, and the sum of the three numbers in each of the two diagonals is always the same number.

In the magic square below, four of the nine numbers are given.

What is the value of $N$?

|  | $\boldsymbol{N}$ |  |
| :--- | :--- | :--- |
| 11 |  | 15 |
| 12 |  | 10 |

(A) 4
(B) 9
(C) 10
(D) 13
(E) 17
A5. Compare the left column with the diagonal in which 10 is located. In the middle box (below the $N$), 13 must be placed because it must hold that $13+10=12+11$. In the middle below, 17 must be placed because $(11+13+15)-(12+10)=17$. Therefore, $N=39-13-17=9$.