Olympiad Maths Prep

Track / Stage 4 / 32 of 340 #292 of 2000

Problem 292

AMC 12 late, AIME early
Combinatorics Difficulty 4.6 Find the answer

6. Let a1,a2,a3,a4a_{1}, a_{2}, a_{3}, a_{4} be any permutation of 1,2,3,41,2,3,4, and ff be a one-to-one mapping from {1,2,3,4}\{1,2,3,4\} to {1,2,3,4}\{1,2,3,4\} such that f(i)if(i) \neq i. Consider the matrix
A=[a1a2a3a4f(a1)f(a2)f(a3)f(a4)]. A=\left[\begin{array}{cccc} a_{1} & a_{2} & a_{3} & a_{4} \\ f\left(a_{1}\right) & f\left(a_{2}\right) & f\left(a_{3}\right) & f\left(a_{4}\right) \end{array}\right] .

If the corresponding positions of two matrices MM and NN differ in at least one entry, then MM and NN are considered two different matrices. The number of different matrices that satisfy the conditions is ( ).
(A) 144
(B) 192
(C) 216
(D) 576

Official solution

6. C.

For a permutation of a1,a2,a3,a4a_{1}, a_{2}, a_{3}, a_{4}, there can be 9 mappings satisfying f(i)if(i) \neq i. Since a1,a2,a3,a4a_{1}, a_{2}, a_{3}, a_{4} have a total of A44=24A_{4}^{4}=24 permutations, therefore, the number of tables satisfying the condition is 24×9=21624 \times 9=216.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.