In the magic-square at the right, what value should X have?

In the magic-square at the right, five more numbers can be placed in the boxes so that the sum of the three numbers in each row, in each column, and in each diagonal is always the same. What value should X have? 

15		35
50
25	X

From the first column, the sum = 15+50+25 = 90

15	A	35
50	B
25	X

From the first row sum = 15+A+35 = 90
=>A = 90-35-15 = 40

From the right diagonal sum = 35+B+25=90
=>B = 90-35-25 = 30

From middle column, sum = A+B+X = 90
=> X = 90-A-B = 90-40-30 = 20

The value of X is 20.