What value should N have?

In the magic square at the right, the four numbers in each column, in each row, and in each of the two diagonals, have the same sum. What value should N have?

For determining the value of N, we need to find the values A,B and C as in the below figure.

Sum of the columns = 12+9+5+8
                                 = 34

7+9+16+A = 34
A = 34-32 = 2

8+11+A+B = 34
B = 34-8-11-2 = 13


12+C+3+B = 34
C = 34-12-3-13 = 6

N+4+9+C = 34
N = 34-4-9-6 = 15

Value of N is 15.