464.All Lines Odd
★★★combinatoricsCh. 2 — Axiomatic and Conditional Probability444+ Problems in Probability
The integers 1–9 are placed uniformly at random on a grid. What is the probability that every row, every column, and both diagonals sum to an odd number?
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Only the parity pattern matters: where can the four even numbers go so that all 8 lines contain an odd count of odd numbers?
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