simple Question on Combinations

Topic : Chess Board Problem

Question : Determine the number of rectangles that can be formed on a chess board.

Answer :

There are nine horizontal lines and there are nine vertical lines.To form a rectangle we need 2 vertical lines and 2 horizontal lines from each set.This can be done in 9C2 *9C2 ways= 36*36= 1296.

Hope the Answer is Helpful.