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counting and probability senior
Problem
There are lily pads in a row numbered to , in that order. There are predators on lily pads and , and a morsel of food on lily pad . Fiona the frog starts on pad , and from any given lily pad, has a chance to hop to the next pad, and an equal chance to jump pads. What is the probability that Fiona reaches pad without landing on either pad or pad ?
(A)
(B)
(C)
(D)
Solution
Firstly, notice that if Fiona jumps over the predator on pad , she must land on pad . Similarly, she must land on if she makes it past . Thus, we can split the problem into smaller sub-problems, separately finding the probability Fiona skips , the probability she skips (starting at ) and the probability she doesn't skip (starting at ). Notice that by symmetry, the last of these three sub-problems is the complement of the first sub-problem, so the probability will be . In the analysis below, we call the larger jump a -jump, and the smaller a -jump. For the first sub-problem, consider Fiona's options. She can either go -jump, -jump, -jump, with probability , or she can go -jump, -jump, with probability . These are the only two options, so they together make the answer . We now also know the answer to the last sub-problem is . For the second sub-problem, Fiona must go -jump, -jump, with probability , since any other option would result in her death to a predator. Thus, since the three sub-problems are independent, the final answer is .
Final answer
A