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Bunuel
What is the number of ways in which 4 squares can be chosen at random on a chess board such that they lie on a diagonal line? (A diagonal line refers to not only the diagonals of the chessboard but rather to all diagonal lines possible on the chessboard)

A. 140
B. 182
C. 256
D. 364
E. 504

Number of diagonals present in a chess board in each direction that can contain 4 squares are:
2 diagonals each of length 4 units, 5, 6, & 7 units and 1 diagonal of length 8 units

Total ways of selecting 4 squares so that they line in a diagonal in each direction = selecting 4 squares from each length
= \(2*4c_4 + 2*5c_4 + 2*6c_4 + 2*7c_4 + 8c_4\)
= \(2(1 + 5 + 15 + 35) + 70\)
= \(182\)

The same number of selections are possible for another direction also
--> Total selections = \(2*182 = 364\)

Option D

Hi @Dilesh4096,

Can you please the logic behind classifying diagonal categories? From my understanding, there are 2 diagonals (left-to-right and right-to-left) with 8 squares in each direction.

Thanks
Only the middle diagonal has 8 squares. As you move towards the corners from the centers, the number of squares goes from 8 to 1 (8,7,6,5,4,3,2,1).
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Bunuel
What is the number of ways in which 4 squares can be chosen at random on a chess board such that they lie on a diagonal line? (A diagonal line refers to not only the diagonals of the chessboard but rather to all diagonal lines possible on the chessboard)

A. 140
B. 182
C. 256
D. 364
E. 504


Are You Up For the Challenge: 700 Level Questions
­I blindly committed a mistake. Chess board is 8x8 so one long diagonal with 8 squares, then 6 more diagonals where the number of squares will be 2,3,4,5,6,7 respectively. We will not consider the squares at the corners. So, to select 4 squares when diagonals are taken from one side, there are 8C4+2*(4C4+5C4+6C4+7C4)=182 ways. I chose option (B) in a hurry forgetting that there is a similar set of diagonals on other side too. So total ways are 182*2=264. Option (D) is correct.­
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