1). 6x-6y=6(x-y);
5x+5a-5b=5(x-a-b);
5a+10b=5(a+2b);
5ab-5a=5a(b-1);
4y-2ay=2y(2y-a);
7a+7b+7c=7(a+b+c);
2). x+y=25, a=4,
a) 7x+7y=7(x+y)=7×25=175;
b) 2x+2y-a=2(x+y)-a=2×25-4=50-4=46;
c) xa-ya=a(x-y)=4×25=100;
7). ab+ac=80;
b+c=16;
a=?
a(b+c)=80;
a×16=80;
a=80:16=5;