2020 AMC 8 Problems/Problem 21
- 1 Problem
- 2 Solution 1
- 3 Solution 2
- 4 Solution 3
- 5 Solution 4: Intuitive Approach
- 6 Video Solution
- 7 Video Solution by OmegaLearn
- 8 Video Solution by MathisCool
- 9 Video Solution by North America Math Contest Go Go Go
- 10 Video Solution by WhyMath
- 11 Video Solutions
- 12 Video Solution by Interstigation
- 13 Video Solution by STEMbreezy
- 14 See also
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.)
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. 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 .
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 .
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
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Video Solution by OmegaLearn
Video Solution by MathisCool
Video Solution by North America Math Contest Go Go Go
~North America Math Contest Go Go Go
Video Solution by WhyMath
Video Solution by Interstigation
Video Solution by STEMbreezy
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