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.