Ten-digit codes that fall then rise
Question
A ten-digit code uses each digit through exactly once. The digits must strictly decrease from to () and then strictly increase from to (). How many such codes are there?
Short answer
. is below every other digit, so . Choosing which 4 of the remaining 9 digits go before it fixes the whole code, since each side has only one valid order. That gives .
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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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