2021 AMC 12A Problems/Problem 23
Contents
- 1 Problem
- 2 Solution 1 (complementary counting)
- 3 Solution 2 (direct counting and probability states)
- 4 Solution 3 (Similar to Solution 2, but Finds the Numerator and Denominator Separately)
- 5 Solution 4
- 6 Solution 5
- 7 Solution 6 (Casework)
- 8 Video Solution by OmegaLearn (Using Probability States)
- 9 See also
Problem
Frieda the frog begins a sequence of hops on a grid of squares, moving one square on each hop and choosing at random the direction of each hop-up, down, left, or right. She does not hop diagonally. When the direction of a hop would take Frieda off the grid, she "wraps around" and jumps to the opposite edge. For example if Frieda begins in the center square and makes two hops "up", the first hop would place her in the top row middle square, and the second hop would cause Frieda to jump to the opposite edge, landing in the bottom row middle square. Suppose Frieda starts from the center square, makes at most four hops at random, and stops hopping if she lands on a corner square. What is the probability that she reaches a corner square on one of the four hops?
Solution 1 (complementary counting)
We will use complementary counting. First, the frog can go left with probability . We observe symmetry, so our final answer will be multiplied by 4 for the 4 directions, and since , we will ignore the leading probability.
From the left, she either goes left to another edge () or back to the center (). Time for some casework.
She goes back to the center.
Now, she can go in any 4 directions, and then has 2 options from that edge. This gives . --End case 1
She goes to another edge (rightmost).
Subcase 1: She goes back to the left edge. She now has 2 places to go, giving
Subcase 2: She goes to the center. Now any move works.
for this case. --End case 2
She goes back to the center in Case 1 with probability , and to the right edge with probability
So, our answer is
But, don't forget complementary counting. So, we get . ~ firebolt360
Video Solution for those who prefer: https://youtu.be/ude2rzO1cmk ~ firebolt360
Solution 2 (direct counting and probability states)
We can draw a state diagram with three states: center, edge, and corner. Denote center by M, edge by E, and corner by C. There are a few ways Frieda can reach a corner in four or less moves: EC, EEC, EEEC, EMEC. Then, calculating the probabilities of each of these cases happening, we have , so the answer is . ~IceWolf10
Solution 3 (Similar to Solution 2, but Finds the Numerator and Denominator Separately)
Denominator
There are ways to make 4 hops without restrictions.
Numerator (Casework)
Suppose Frieda makes 4 hops without stopping. We perform casework on which hop reaches a corner for the first time.
(1) Hop #2 (Hops #3 and #4 have no restrictions)
The 4 independent hops have 4, 2, 4, 4 options, respectively. So, this case has ways.
(2) Hop #3 (Hop #4 has no restriction)
No matter which direction the first hop takes, the second hop must "wrap around".
The 4 independent hops have 4, 1, 2, 4 options, respectively. So, this case has ways.
(3) Hop #4
Two sub-cases:
(3.1) The second hop "wraps around". It follows that the third hop also "wraps around".
The 4 independent hops have 4, 1, 1, 2 options, respectively. So, this sub-case has ways.
(3.2) The second hop backs to the center.
The 4 independent hops have 4, 1, 4, 2 options, respectively. So, this sub-case has ways.
Together, Case (3) has ways.
The numerator is
Probability
This problem is quite similar to 1995 AIME Problem 3: https://artofproblemsolving.com/wiki/index.php/1995_AIME_Problems/Problem_3
~MRENTHUSIASM
Solution 4
Let be the probability that Frieda is on the central square after n moves, be the probability that Frieda is on one of the four squares on the middle of the edges after n moves, and (V for vertex) be the probability that Frieda is on a corner after n moves. The only way to reach the center is by moving in specific direction out of total directions from the middle of an edge, so . The ways to reach the middle of an edge are by moving in any direction from the center or by moving in specific direction from the middle of an edge, so . The ways to reach a corner are by simply staying there after reaching there in a previous move or by moving in specific directions from the middle of an edge, so . Since Frieda always start from the center, , , and . We use the previous formulas to work out and find it to be .
-SmileKat32
Solution 5
Imagine an infinite grid of by squares such that there is a by square centered at for all ordered pairs of integers
It is easy to see that the problem is equivalent to Frieda moving left, right, up, or down on this infinite grid starting at . (minus the teleportations) Since counting the complement set is easier, we'll count the number of -step paths such that Frieda never reaches a corner point.
In other words, since the reachable corner points are and Frieda can only travel along the collection of points included in , where is all points on and such that and , respectively, plus all points on the big square with side length centered at We then can proceed with casework:
Case : Frieda never reaches nor
When Frieda only moves horizontally or vertically for her four moves, she can do so in ways for each case . Thus, total paths for the subcase of staying in one direction. (For instance, all length combinations of and except , , , and for the horizontal direction.)
There is another subcase where she changes directions during her path. There are four symmetric cases for this subcase depending on which of the four quadrants Frieda hugs. For the first quadrant, the possible paths are , , , and Thus, a total of ways for this subcase.
Total for Case :
Case : Frieda reaches or .
Once Frieda reaches one of the points listed above (by using three moves), she has four choices for her last move. Thus, a total of paths for this case.
Our total number of paths never reaching coroners is thus making for an answer of
-fidgetboss_4000
Solution 6 (Casework)
We take cases on the number of hops needed to reach a corner. For simplicity, denote as a move that takes Frieda to an edge, as wrap-around move and as a corner move. Also, denote as a move that takes us to the center.
2 Hops
Then, Frieda will have to as her set of moves. There are ways to move to an edge, and corners to move to, for a total of cases here. Then, there are choices for each move, for a probability of .
3 Hops
In this case, Frieda must wrap-around. There's only one possible combination, just . There are ways to move to an edge, way to wrap-around (you must continue in the same direction) and corners, for a total of cases here. Then, there are choices for each move, for a probability of .
4 Hops
Lastly, there are two cases we must consider here. The first case is , and the second is . For the first case, there are ways to move to an edge, way to return to the center, ways to move to an edge once again, and ways to move to a corner. Hence, there is a total of cases here. Then, for the second case, there are ways to move to a corner, way to wrap-around, way to wrap-around again, and ways to move to a corner. This implies there are cases here. Then, there is a total of cases, out of a total of cases, for a probability of .
Then, the total probability that Frieda ends up on a corner is , corresponding to choice . ~rocketsri
Video Solution by OmegaLearn (Using Probability States)
~ pi_is_3.14
See also
2021 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 22 |
Followed by Problem 24 | |
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 10 Problems and Solutions |
2021 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 22 |
Followed by Problem 24 |
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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