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i forgot a basic gdt thing, pls help me

Hello i ve question about positon
Hole should have position of X 0 and Y0, measured position is X0.15, Y0
measured position is 0.3. (double value of 0.15)
i know how to calculate position sqrt(x.dev^2+y.dev^2) x 2
but i need a reminder why that is
please