9.15 In how many ways can two rooks be placed on a chessboard so that one cannot capture the other? (One rook can capture another if it is on the same row or the same column of the chessboard).

## Group 5

Solve the equations (9.16-9.17):
9.15 The first rook can be placed on any of the 64 squares. In this case, 14 squares are under threat, so for the second rook, there are any of $64-15=49$ squares left. Thus, the total number of options is $64 \cdot 49=3136$.

Answer: 3136 ways.