We have the rectangle with dimensions 10 ×11 (10 horizontal dots and 11 vertical). PQ is parallel to y-axis and PR is parallel to x-axis.
Choose the (x, y) coordinates for vertex P (right angle): 10C1 × 11C1
Choose the x coordinate for vertex R (as y coordinate is fixed by A): 9C1, (10 – 1 = 9 as 1 horizontal dot is already occupied by A)
Choose the y coordinate for vertex Q (as x coordinate is fixed by A): 10C1, (11 – 1 = 10 as 1 vertical dot is already occupied by A).
Hence, required number of triangles will be 10C1 × 11C1 × 9C1 × 10C1 = 9900.