Tic Tac Toe
Math brain teasers require computations to solve.
Arrange the numbers 1 through 9 on a tic tac toe board such that the numbers in each row, column, and diagonal add up to 15.
Answer
4, 3, 8,
9, 5, 1,
2, 7, 6,
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Comments
dsquared
Jul 22, 2002
 Nice one... many possible scramblings of the answer too ;P 
xiaobaby
Aug 10, 2002
 rather simple.. just have 2,4,6 and 8 anywhere at the corners and 5 in the middle and u can easily solve it 
sillywilly
Aug 10, 2002
 This one was really fun because there are so many ways to solve it, and many ways to get the "aha!"  you can check for the center (same number, 4 combos  only 5 works), or do the big numbers (987 need to be on different rows/columns and not on a diag)  on and on. very good fun. 
pemalova
Aug 21, 2002
 I got that in like 5 minutes. Is that good? Dang. I sound like a girl. Oh well....there's nothing wrong with being in touch with my feminine side is there? 
SgtScoobz
Apr 05, 2003
 a little easy for my tastes because we do that stuff in math all of the time 
gcriotgurl
Aug 10, 2003
 this was pretty boring...sorry but it was. i've had to do this i don't know how many times in school so far for EXTRA CREDIT...pretty lame. 
luvupurple
Aug 10, 2003
 GOOD 1! luvupurple 
JmBannana
Aug 11, 2004
 I also have to say it was pretty easy but a good teaser 
Skybet
Nov 20, 2004
 Takes prettly long to work out, but time consuming and fun, good job 
deadeye823
Apr 10, 2005
 ive done this before, you can even do boxes of four by four but i forget what they have to add up to... 
lilredc5
Jun 06, 2007
 Hmm.. Answer maybe should have indicated that there are several other answers, and maybe some reasoning behind it. The people that said it is easy say that because they got lucky in their trialanderror. No one mentioned any logic, like 1 can't go with 2 or 3, 7 can't go with 4, 9 can't go with 3 or 68, and that 5 is the only number w/o any restrictions and thusly must go in the middle, since every other square forms a sum somehow with the middle square. 
pating
Aug 31, 2007
 I was told of the solution to this problem before when I was a child. But it only works if the box is an odd number, meaning its 3x3, 5x5, 7x7, etc...
Firstly, you can draw a grid, say 5x5 grid.
Then place the 1 in the rightmost column, middle row.
Next step is, go 1 box down, 1 box to right, then you place the next number (2) in that box.
But if you're on the rightmost column, and you need to go 1 box to the right, you'll have to go 4 boxes to left instead (or go to the leftmost box in the same row).
And if you need to go 1 box below and you're in the bottom row, you'll have to go 4 boxes above instead (or go to the topmost box in the same column).
You'll notice that if you're already in the number 5, if you go 1 box down and 1 box to right, that box is already occupied by number 1. So instead, go 1 box to left and place the next number (6) there. Then repeat the process.
Here is the solution for a 5x5 grid.
11 10 4 23 17
18 12 6 5 24
25 19 13 7 1
2 21 20 14 8
9 3 22 16 15
The sums of the columns, rows and diagonal are all 65.
You can try it in a 7x7 grid, the sums should be 175!

pating
Aug 31, 2007
 11_10__4_23_17
18_12__6__5_24
25_19_13__7__1
_2_21_20_14__8
_9__3_22_16_15 
javaguru
Jan 04, 2009
 Very nice solution pating. I think I saw that before, but had forgotten about it.
The sum for each row/column/diagonal in an NxN magic square is (NxN+1) x N/2. 
opqpop
Sep 10, 2010
 The middle number requires 4 sets of pairs with equal sum. Hence, 5 is the only possible middle number. From there, it's casework with 91 being a sum in a diagonal or nondiagonal. Use the 9 to figure out where 8 can go, and you eventually arrive at the only case that works. 
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