Random rectangle hitting the middle row of a grid
Question
In a grid of unit squares, all squares in the middle row (row ) 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
. Rectangles are pairs of horizontal and pairs of vertical grid lines. It misses the shaded row only if both horizontal lines lie among the above it or the below it, probability ; the complement is .
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Open the worked solutionLessons behind this question
- Counting and Combinatorics
- Sample Spaces, Events, and the Probability Axioms
- Conditional Probability, Total Probability, and Bayes' Rule
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