2023 AMC 12A Problems/Problem 17
Problem
Flora the frog starts at 0 on the number line and makes a sequence of jumps to the right. In any one jump, independent of previous jumps, Flora leaps a positive integer distance with probability
.
What is the probability that Flora will eventually land at 10?
Solution 1
Let's denote as the probability of reaching
from
. We immediately see that
, and
, since there's only one way to get to 1 from 0. Just jump!
Now, let's write an expression for . Suppose we know
.
The probability of reaching 10 from some integer will be
(use the formula given in the problem!)
The probability of reaching that integer
from
is going to be
. Then, the probability of going from
will be
We want the probability of reaching 10 from anywhere though, so what we can do is sum over all passing points
, i.e.
Now that we have expressed our problem formally, we can actually start solving it!
Let's calculate (our expression is actually a general fact, not just limited to
).
Hmm, we see that the first 8 terms of
are exactly the first 8 terms of
. Let's substitute it in.
Isn't that interesting. Turns out, this reasoning can be extended all the way to
.
It breaks at , since
. Anyway, with this, we see that the answer is just
~
Solution 2
In order to find the probability of landing on 10, we must multiply the amount of successful combinations by the probability of those combinations. Notice for any successful combination of steps, the probability must always be . Now, we only need to find the amount of possibilites for steps since we know the probaility of each combination occuring is the same. This can be done using sticks and stones (9C0+9C1+9C2+9C3+...9C9) = 2^(9). This the final answer is
multiplied by
or
~ShangJ2
Solution 3 (engineer's induction)
The probability frog lands on 1 is trivially
The probability frog lands on 2 is $\frac{1}{4} + \frac{1}{2}\cdot \frac{1}[2} = \frac{1}{2},$ (Error compiling LaTeX. Unknown error_msg) from the two cases 0-2 and 0-1-2.
The probability frog lands on 3 is
from the cases 0-3, 0-1-3 and 0-2-3, 0-1-2-3.
The probability frog lands on 4 is
from the cases 0-4, 0-1-4 and 0-3-4, 0-2-4, 0-1-2-4 and 0-1-3-4 and 0-2-3-4, 0-1-2-3-4.
It looks like the probability is
regardless of the ending number. Therefore, we choose
~sirswagger21
See also
2023 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 16 |
Followed by Problem 18 |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
All AMC 12 Problems and Solutions |
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