How Many Rectangles in the Grid?

Explanation

Count all rectangles in a 3x3 grid using the combination formula. A rectangle is defined by choosing 2 vertical lines from the 4 available and 2 horizontal lines from the 4 available: C(4,2) x C(4,2) = 6 x 6 = 36. This includes all sizes: 1x1, 1x2, 1x3, 2x1, 2x2, 2x3, 3x1, 3x2, and 3x3 rectangles.