[tex]b=a+1\\\\a=5k+3[/tex] gdzie [tex]k\in\mathbb{Z}[/tex]
[tex]3(a+b)=3(a+a+1)=6a+3[/tex]
a)
[tex]6a+3=6(5k+3)+3=30k+18+3=30k+21[/tex]
Reszta to 21.
b)
[tex]30k+21=30k+15+6=15(2k+1)+6[/tex]
Reszta to 6.
c)
[tex]30k+21=30k+20+1=10(3k+2)+1[/tex]
Reszta to 1.