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Six random pieces of a stick all at most half

Question

A stick of length 11 is cut at 55 points chosen independently and uniformly along it, producing 66 pieces. What is the probability that none of the pieces is longer than 12\tfrac12?

Short answer

1316\tfrac{13}{16}. At most one piece can exceed 12\tfrac12, so the bad events are disjoint. A given piece exceeds 12\tfrac12 with probability (1−12)5=132(1-\tfrac12)^{5}=\tfrac1{32} (the spacings are exchangeable, so each piece behaves like the first). Hence the answer is 1−6⋅132=13161-6\cdot\tfrac1{32}=\tfrac{13}{16}.

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

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