设这个三位数为100x+10y+z,则
x=2y+1
z=3y-1
(100z+10y+x)-(100x+10y+z)=A
-99(x-z)=A
将1式-2式,得
x-z=-y+2
代入
99(y-2)=A>0
y>2
2y+1