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For point P, x & y can be chosen in 10*11 ways (10 possible x coord. values. 11 possible Y coordinate values. Once P is chosen, the y coordinate of R is fixed since PR is || to X axis. Therefore x coordinate for R can be one of (10 - 1) = 9 values. -1 because it cannot be same as P.
Similarly, Q has same X coordinate as P, but Y coordinate could be any of (11 - 1) values.
therefore total number of right angled triangles = 10*11*9*10 = 9,900