Tic-tac-toe stranger

Probability Level pending

Draw an X or an O into each empty cell to the figure below, such that there mustn't be four X's or four O's in the same row/column/diagonal one after an other.

If you finished, then calculate the number of X's!

Here is a sample:

27 29 25 28 26

2 solutions

Marco Brezzi
Aug 20, 2017

Suppose we put an $X$ in that spot, after some step following the rules we get to this

Consider the green square:

if we put an $X$ , we break the rules (horizontally)
if we put an $O$ , we break the rules (diagonally)

So the initial $X$ we put should have been an $O$

Following the rules we arrive to the final configuration

Which has $\boxed{26}$ $X$ 's

Áron Bán-Szabó
Aug 20, 2017

Tic-tac-toe stranger

2 solutions

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