Difference between revisions of "2020 AMC 8 Problems/Problem 21"
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==Solution 1== | ==Solution 1== | ||
− | Notice that, in order to step onto any particular white square, the marker must have come from one of the <math>1</math> or <math>2</math> white squares immediately beneath it (since the marker can only move on white squares). This means that the number of ways to move from <math>P</math> to that square is the sum of the numbers of ways to move from <math>P</math> to each of the white squares immediately beneath it. To solve the problem, we can accordingly construct the following diagram, where each number in a square is calculated as the sum of the numbers on the white squares immediately beneath that square (and thus will represent the number of ways to remove from <math>P</math> to that square, as already stated). | + | Notice that, in order to step onto any particular white square, the marker must have come from one of the <math>1</math> or <math>2</math> white squares immediately beneath it (since the marker can only move on white squares). This means that the number of ways to move from <math>P</math> to that square is the sum of the numbers of ways to move from <math>P</math> to each of the white squares immediately beneath it(also called the Water Fall Method). To solve the problem, we can accordingly construct the following diagram, where each number in a square is calculated as the sum of the numbers on the white squares immediately beneath that square (and thus will represent the number of ways to remove from <math>P</math> to that square, as already stated). |
<asy> | <asy> | ||
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The answer is therefore <math>\boxed{\textbf{(A) }28}</math>. | The answer is therefore <math>\boxed{\textbf{(A) }28}</math>. | ||
+ | |||
+ | ~ edited by Dream9 | ||
==Solution 2== | ==Solution 2== |
Latest revision as of 22:50, 19 January 2024
Contents
- 1 Problem
- 2 Solution 1
- 3 Solution 2
- 4 Solution 3
- 5 Solution 4: Intuitive Approach
- 6 Video Solution by Math-X (First understand the problem!!!)
- 7 Video Solution (🚀 Just 2 min 🚀)
- 8 Video Solution
- 9 Video Solution by OmegaLearn
- 10 Video Solution by MathisCool
- 11 Video Solution by North America Math Contest Go Go Go
- 12 Video Solution by WhyMath
- 13 Video Solutions
- 14 Video Solution by Interstigation
- 15 Video Solution by STEMbreezy
- 16 See also
Problem
A game board consists of squares that alternate in color between black and white. The figure below shows square
in the bottom row and square
in the top row. A marker is placed at
A step consists of moving the marker onto one of the adjoining white squares in the row above. How many
-step paths are there from
to
(The figure shows a sample path.)
Solution 1
Notice that, in order to step onto any particular white square, the marker must have come from one of the or
white squares immediately beneath it (since the marker can only move on white squares). This means that the number of ways to move from
to that square is the sum of the numbers of ways to move from
to each of the white squares immediately beneath it(also called the Water Fall Method). To solve the problem, we can accordingly construct the following diagram, where each number in a square is calculated as the sum of the numbers on the white squares immediately beneath that square (and thus will represent the number of ways to remove from
to that square, as already stated).
The answer is therefore .
~ edited by Dream9
Solution 2
Suppose we "extend" the chessboard infinitely with additional columns to the right, as shown below. The red line shows the right-hand edge of the original board.
The total number of paths from to
, including invalid paths which cross over the red line, is then the number of paths which make
steps up-and-right and
steps up-and-left, which is
. We need to subtract the number of invalid paths, i.e. the number of paths that pass through
or
. To get to
, the marker has to make
up-and-right steps, after which it can proceed to
with
steps up-and-left and
step up-and-right. Thus, the number of paths from
to
that pass through
is
. Similarly, the number of paths that pass through
is
. However, we have now double-counted the invalid paths which pass through both
and
; from the diagram, it is clear that there are only
of these (as the marker can get from
to
by a step up-and-left and a step-up-and-right in either order). Hence the number of invalid paths is
, and the number of valid paths from
to
is
.
Solution 3
On any white square, we may choose to go left or right, as long as we do not cross over the border of the board. Call the moves and
respectively. Every single legal path consists of
and
, so now all we have to find is the number of ways to order
and
in any way, which is
. However, we originally promised that we will not go over the border, and now we have to subtract the paths that do go over the border. The paths that go over the border are any paths that start with RRR(1 path), RR(5 paths) and LRRR(1 path) so our final number of paths is
~PEKKA
Solution 4: Intuitive Approach
We label the rows starting from the bottom. At row 1, there is way: at P. We draw all the possible ways to get to Q. There are two ways to choose for row 2, and another two ways to choose for row 3. However, you can go to the "edge" or the farthest possible square westward of Q, so you can't multiply by 2 again. Notice how, at the first step, we figured that the answer was even, so choice D and E are eliminated, and after the second row, we realized it must be a multiple of 4, so choice B is eliminated. When we get to the fourth row, we do not multiply by 2 again, since we have limited possibilities rather than multiplying by 2 again. Choice C implies that there are two possibilities per row; however, we know that if you go to the farthest possible, you only have one possibility, so it is not
so we know that the answer is choice
.
~hh99754539
Video Solution by Math-X (First understand the problem!!!)
https://youtu.be/UnVo6jZ3Wnk?si=X5JGwbGqL4dqK6t0&t=4098
~Math-X
Video Solution (🚀 Just 2 min 🚀)
~Education, the Study of Everything
Video Solution
Please like and subscribe!
Video Solution by OmegaLearn
https://youtu.be/CNgHB-bsqFk?t=1441
~ pi_is_3.14
Video Solution by MathisCool
https://www.youtube.com/watch?v=LvpCYn8Ze6c
Video Solution by North America Math Contest Go Go Go
https://www.youtube.com/watch?v=rWxhlItMAN0
~North America Math Contest Go Go Go
Video Solution by WhyMath
~savannahsolver
Video Solutions
Video Solution by Interstigation
https://youtu.be/YnwkBZTv5Fw?t=1247
~Interstigation
Video Solution by STEMbreezy
~STEMbreezy
See also
2020 AMC 8 (Problems • Answer Key • Resources) | ||
Preceded by Problem 20 |
Followed by Problem 22 | |
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 AJHSME/AMC 8 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.