Rhombus Grid

To go from A A to B B , you only can move along \nearrow and \searrow and you can't pass through more than one star.

How many paths are there to go from A A to B B ?


The answer is 30.

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2 solutions

Jesse Nieminen
Jun 27, 2016

Relevant wiki: Rectangular Grid Walk

We can obtain the number of paths easily by using this method:

Give every dot a value of 0 0 .
Add 1 1 to A A :s value.
Add A A :s value to the dots to which can be moved from A A .
Repeat the last step for every new dot you add to until you get to B B .
The number of paths is the value of B B .


Let's divide the problems into 3 3 separate cases.

Case 1: Passing trough no stars
Imagine that the dots with a star don't exist and calculate the number of possible paths which is 6 6 .

Case 2: Passing trough the first star
Imagine that the dots above and below the first star don't exist (forcing the path go through the first star) and calculate the number of possible paths which is 12 12 .

Case 3: Passing trough the last star
By symmetry the number of paths is also 12 12 .

Adding the values we obtain the answer: 6 + 12 + 12 = 30 6 + 12 + 12 = \boxed{30} .

You could also calculate the number of all paths and then subtract the number of paths which go through both stars. This also gives the same answer

Thanks. I've updated the answer from 36 to 30. Those who previously answered 30 will be marked correct, while those who previously answered 36 will be marked wrong.

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Brilliant Mathematics Staff - 4 years, 11 months ago

Just a matter of waiting out rounds in the roster. If you count them with passing through the first star, you May double your answer because of the symmetry. So op the outside you got 1's. Gettin to the star makes 2, from the star to outside makes 3 routes above and 3 routes down. You got 6 routes to middle. Since you can't pass trough the 2nd star, there are 9 rounds around via the top and 9 via the downside. Hence 18 routes from A to B through the first star. This makes 36 routes in total.

By mirroring the 18, you are counting 6 paths twice. The 6 paths that don't go through any of the stars.

Romain Farthoat - 4 years, 11 months ago

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So it would make 35 routes then? Or am I counting one path twice and am i forgetting another route?

Peter van der Linden - 4 years, 11 months ago

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Nope, 30. From A to B there are 6 paths that do not pass through any star, and you are counting these paths twice!

Romain Farthoat - 4 years, 11 months ago

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@Romain Farthoat Hmmm it Does give me a correct when i answer 36, so i am gonna rethink :) And rethinking helped: Just took al the paths from A to B and took out those that passed through both the stars: 54 - 2 x 6 x 2 = 30

Peter van der Linden - 4 years, 11 months ago

I drew a first grid that passed through the first star, with A A paths, a second grid that passed through the second star with B B paths then I drew a grid that didn't pass trough any star with A B A \cap B paths. By using Pascal's Triangle, I calculated : ( A ) + ( B ) ( A B ) 18 + 18 6 = 30 ( A ) + ( B ) - ( A \cap B ) \leftrightarrow 18 + 18 - 6 = \boxed{ 30 } I don't know why this is level 5 honestly. The strange thing is that the answer 30 was not accepted.

Armand Diamond - 4 years, 11 months ago

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That is why i didnt see my double count

Peter van der Linden - 4 years, 11 months ago

It is definitely 30 paths.

Mohammed Amr - 4 years, 11 months ago

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