2014 AIME I Problems/Problem 12

Problem 12

Let $A=\{1,2,3,4\}$ , and $f$ and $g$ be randomly chosen (not necessarily distinct) functions from $A$ to $A$ . The probability that the range of $f$ and the range of $g$ are disjoint is $\tfrac{m}{n}$ , where $m$ and $n$ are relatively prime positive integers. Find $m$ .

Solution

We note there are $4^8$ sets of two functions $f$ and $g$ from $A$ to $A$ since the input of the four values of each function has four options each By the pigeonhole principle the combined range of $f$ and $g$ has at most four elements this can be done in 3 cases:

Case 1: $f$ 's range contains 3 elements

there are 4 ways to choose the range of $f$ ${{4}\choose {3}}$ then three elements of A can be assigned in $0.5*3*4*(3!)$ ways: choose which element will be assigned to more than once (3) how three elements will be assigned to to three elements of A(6) which of the four elements will be assigned to the extra value, divided by 2 becuase of over-counting

note that $g$ can only be the function assigning each element of $A$ to the element not already chosen

so there $4*36$ for that to happen

Case 2: $f$ 's range contains 2 elements

there are 6 ways to choose the range of $f$ ${{4}\choose {2}}$ then each element in $f$ can be assigned to one of the 2 elements in the range of $f$ so there are $4*2^4$ functions with a range of 3 elements in $A$

now $g$ can have a range of two elements in which each of its 4 values can be assigned one of its elements ( $2^4-2$ ways)

or $g$ can have a range of one element there are 2 ways to choose its range ${{2}\choose {1}}$ and like in case 1 $g$ can happen in exactly 1 way

so there $(6*(2^4-2))(2^4)$ ways for that to happen

case 3: $f$ 's range contains 1 element

here are 4 ways to choose the range of $f$ ${{4}\choose {1}}$ then each element in $A$ can be assigned to only one value so there are 4 functions with a range of 1 elements in $A$

now $g$ can have a range of 1,2 or 3: if $g$ 's range has 3 elements each value in $A$ can be assigned to 3 other values so there $3^4$ ways for that to occur.

if $g$ 's range contains 2 elements one can choose the 2 elements in 3 different ways ${{3}\choose {2}}$ and after choosing each element has 2 options resulting with $2^4$ ways for that to occur

if $g$ 's range contains 1 element one can choose the 1 elements in 3 different ways ${{3}\choose {1}}$ and after choosing each element has 1 option resulting with 1 way for that to happen. so there are $4*(3^4)$ ways for that to occur

summing the cases

we get that the probability for $f$ and $g$ to have disjoint ranges is equal to:

$\dfrac{4*3^4+(6*2^4)(2^4-2)+4*(36)}{4^8} =\dfrac{2^2*3^4+2^6*3*7+2^4*3^2}{2^{16}}= \dfrac{3^4+2^4*3*7+3^2*2^2}{2^{14}}=\dfrac{453}{2^{14}}$ so the final answer is $453$

2014 AIME I (Problems • Answer Key • Resources)
Preceded by Problem 11	Followed by Problem 13
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
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2014 AIME I Problems/Problem 12

Problem 12

Solution

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