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Random rectangle hitting the middle row of a grid

Question

In a 2023×20232023\times2023 grid of unit squares, all squares in the middle row (row 10121012) are shaded. A rectangle made of whole grid squares is chosen uniformly at random among all such rectangles. What is the probability it includes at least one shaded square?

Short answer

10122023\tfrac{1012}{2023}. Rectangles are pairs of horizontal and pairs of vertical grid lines. It misses the shaded row only if both horizontal lines lie among the 10121012 above it or the 10121012 below it, probability 2(10122)(20242)=10112023\tfrac{2\binom{1012}{2}}{\binom{2024}{2}}=\tfrac{1011}{2023}; the complement is 10122023\tfrac{1012}{2023}.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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