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AMO 2027 Grades 9-11 SET 4 - Self-study Question 10

AMO · AMO 2027 Grades 9-11 SET 4 - Self-study

Question 10

Seven distinct awards are to be handed to seven students, one award per student, with one intended recipient for each award. Assume all $7!$ assignments are equally likely. The probability that every award goes to an incorrect recipient is $\frac ab$ in lowest terms. Find $a+b$. The corresponding probabilities with five and six awards are given as $\frac{11}{30}$ and $\frac{53}{144}$, respectively.

Enter one positive integer, without units.