# Math Help - probabilty of choosing squares in diagonal

1. ## probabilty of choosing squares in diagonal

Hello ,
This my first post here.

Two blocks are chosen randomly on a chess board. What is the probability that they lie on the same diagonal?

2 blocks can be chosen in 64C2 ways .
now we have to choose 2 blocks that lie in the same diagonal.
this can be done in 2 x 8C2 ways (as there are 2 diagonals)
so the probability is 56/2016. this is wrong according to the book.
Thank you

2. Originally Posted by kannan
Hello ,
This my first post here.

Two blocks are chosen randomly on a chess board. What is the probability that they lie on the same diagonal?
Are you sure they are talking only about the main diagonal?

3. Originally Posted by Isomorphism
Are you sure they are talking only about the main diagonal?
oh i must have gone wrong there and is this answer right if they are talking about the main diagonal?
Thank you very much

4. Originally Posted by kannan
oh i must have gone wrong there and is this answer right if they are talking about the main diagonal?
Yes for the 'main diagonal' that is correct.