Magic 3 by 3 Box

 At the beginning I know that I have to put the 5 in the middle of the box since you need to have the number in the middle so it kind of balances the sum of each row, column and diagonal . With the 5 put down in the middle, I know that the other two entries of each row, column and diagonal need to add up to 10. That left us with 4 pairs of numbers to put in (1 and 9, 2 and 8, 3 and 7, 4 and 6, shown in step 1 in the picture). 

Next I just do trial and error. My first trial I put 1 and 9 on the diagonal (shown in second step), and I found out that 9 cannot be in the diagonal since it will come in 'contact' with too many other numbers and the problem with it is it is too big of a number. Since it is too big, the sum for certain row, column or diagonal may go over 15. So it is best that the 9 tug in middle column or the middle row where it comes into 'contact' with as few other numbers as possible. 

For my answer, I put 9 in the middle entry of the bottom row. So since it is a big number, the other 2 entries must add up to 6, and in this case you can only have 2 and 4. Once I have filled in the 2 and 4, the other numbers just fill in pretty smoothly. In the end, my answer is:

8 1 6

3 5 7

4 9 2