1.
[tex]B=\overline{5xy}+\overline{x3y}+\overline{xy7}=\\
B=500+10x+y+100x+30+y+100x+10y+7\\
B=537+210x+12y=3(179+70x+4y) \Rightarrow B\;\vdots \; 3[/tex]
2.
[tex]C=\overline{xy}+\overline{yz}+\overline{zx}\\
C=10x+y+10y+z+10z+x=11x+11y+11z\\
C=11(x+y+z)\Rightarrow C \; \vdots \; 11[/tex]