# 2019 AIME II Problems/Problem 2

## Problem 2

Lily pads $1,2,3,\ldots$ lie in a row on a pond. A frog makes a sequence of jumps starting on pad $1$. From any pad $k$ the frog jumps to either pad $k+1$ or pad $k+2$ chosen randomly with probability $\tfrac{1}{2}$ and independently of other jumps. The probability that the frog visits pad $7$ is $\tfrac{p}{q}$, where $p$ and $q$ are relatively prime positive integers. Find $p+q$.

## Solution

Let $P_n$ be the probability the frog visits pad $7$ starting from pad $n$. Then $P_7 = 1$, $P_6 = \frac12$, and $P_n = \frac12(P_{n + 1} + P_{n + 2})$ for all integers $1 \leq n \leq 5$. Working our way down, we find $$P_5 = \frac{3}{4}$$ $$P_4 = \frac{5}{8}$$ $$P_3 = \frac{11}{16}$$ $$P_2 = \frac{21}{32}$$ $$P_1 = \frac{43}{64}$$ $43 + 64 = \boxed{107}$.

## Solution 2(Casework)

Define a one jump to be a jump from k to K + 1 and a two jump to be a jump from k to k + 2.

Case 1: (6 one jumps) (1/2)^6 = 1/64

Case 2: (4 one jumps and 1 two jumps) 5C1 x (1/2)^5 = 5/32

Case 3: (2 one jumps and 2 two jumps) 4C2 x (1/2)^4 = 3/8

Case 4: (3 two jumps) (1/2)^3 = 1/8

Summing the probabilities gives us 43/64 so the answer is 107.

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