Mock AIME 1 2007-2008 Problems/Problem 8

Problem

A sequence of ten $0$ s and/or $1$ s is randomly generated. If the probability that the sequence does not contain two consecutive $1$ s can be written in the form $\dfrac{m}{n}$ , where $m,n$ are relatively prime positive integers, find $m+n$ .

Solution

Let $a_n$ denote the number of sequences of length $n$ that do not contain consecutive $1$ s. A sequence of length $n$ must either end in a $0$ or a $1$ . If the string of length $n$ ends in a $0$ , this string could have been formed by appending a $0$ to any sequence of length $n-1$ , of which there are $a_{n-1}$ such strings. If the string of length $n$ ends in a $1$ , this string could have been formed by appending a $01$ (to avoid consecutive $1$ s) to any sequence of length $n-2$ , of which there are $a_{n-2}$ such strings. Thus, we have the recursion $a_n = a_{n-1} + a_{n-2}$ Solving for initial conditions, we find $a_1 = 2, a_2 = 3$ . Thus we have the Fibonacci sequence with shifted indices; indeed $a_n = F_{n+2}$ , so $a_{10} = F_{12} = 144$ . The probability is $\frac{144}{2^{10}} = \frac{9}{64}$ , and $m+n=\boxed{073}$ .

Mock AIME 1 2007-2008 (Problems, Source)
Preceded by Problem 7	Followed by Problem 9
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15

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Mock AIME 1 2007-2008 Problems/Problem 8

Problem

Solution

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