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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).
10C1*11C1*9C1*10C1=9900.