How many rectangles are there in the following figure? (Squares are also rectangles)
The above figure contains 7 vertical and 6 horizontal lines. Selecting any 2 of the vertical and 2 of the vertical lines will give a unique enclosed rectangle.
This can be done in $$^7C_2\times\ ^6C_{2\ }=\ 21\times\ 15\ =\ 315$$
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