Can someone find a solution of this problem....

I have 500mmX500mm rhombus ( Rhombus A ). This is divided into 2500 equal size rhombuses of size 10mmX10mm ( R1 - R2500) . Now what I need to find is if I place a point(x,y) in Rhombus A, I have to locate in which of the rhombus of R1-R2500 the point(x,y) is placed.

Thanks in advance