XYn-0.2(XYn-n)=XYZ
XYn-0.2(XY-1)n=XYZ
(XY-0.2(XY-1))n=XYZ
(0.8XY+0.2)n=XYZ
n=XYZ/(0.8XY+0.2)
n=10XYZ/(8XY+2)
n=5XYZ/(4XY+1)

のうちで好きな式にぶちこむ