The key I would think is to find the answer without having to complete the magic square.
Let us call the funny symbol as X (the answer we need to find).
One can see that the first column (which has X) and the third row share a common cell. Let us call it C.
In which case, the sum of first column is = 14 + X + C + 15 = 29 + X + C
Sum of the third row is = C + 5 + 9 + 16 = 30 + C
As it is a magic square, the sum of first column and third row need to be equal to each other i.e.
29 + X + C = 30 + C
X is therefore equal to 1