A probability problem by Nico Stirling
Some coins are placed on a 20 × 13-board. Two coins are called neighbors if they are in the same row or column and no other coins between them. What is the largest number of coins that can be placed on the board if no coin is allowed to have more than two neighbors?
The answer is 33.
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1 solution
now we can take a 20 X 13 board in that there is only one possibility to arrange that is along one side of the board and that of the other side you can see it in the image .
in the image 1 is that there is one coin in that position therefore total no of coins that can be arranged is 21 + 13 - 1 = 33 iam subracting 1 because there is a common coin in the corner of the box sorry you can see a unknown 1 in the middle of the board dont consider that it is the cursor