Math Help?

Question

In the magic square below, 5 more numbers can be placed so that the sum of the three numbers in any row, column, or diagonal will be the same. What value should X have?

15      A     35

50      B     C

25      X     D

Answer 1

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If you look at the first column, you have a sum of 90, so all the others must have the same sum of 90.

Now look at the first row:

15 + A + 35 = 90

A + 50 = 90

A = 40

Then look at the diagonal going up to the right:

25 + B + 35 = 90

B + 60 = 90

B = 30

Finally look at the middle column:

A + B + X = 90

40 + 30 + X = 90

X + 70 = 90

X = 20

Answer:

X = 20 <--

P.S. You could figure out C and D easily but it isn't necessary to find the value of X. If you did, the final square would be:

15 40 35

50 30 10

25 20 45

If you divide each number by 5 and subtract 1, you get a more typical magic square using the digits 1 to 9:

2 7 6

9 5 1

4 3 8

Answer 2

15 + 50 + 25 = 90

15 + 35 = 50, so A = 40

25 + 35 = 60, so B = 30

40 + 30 = 70, so X = 20

check 

25 + 20 = 45, so D = 45

50 + 30 = 80, so C = 10

35 + 10 + 45 = 90

15 + 30 + 45 = 90

Ans X = 20