1) a-3b= -30
a+b=26
(a-3b)-(a+b)=-30-26
a-3b-a-b=-56
-4b=-56
b=-56:-4
b=14
a+14=26
a=26-14
a=12
2) a/b=2/5
a+30b/100=14
a=2b/5
2b/5+30b/100=14
20x2b+30b=14x100
40b+30b=1400
70b=1400
b=1400:70
b=20
a=2x20:5
a=40:5
a=8
3) a/b=7/2
3a-b/4=123
a=7b/2
3x7b/2-b/4=123
21b/2-b/4=123
21bx2-b=123x4
42b-b=492
41b=492
b=492:41
b=12
a=7x12:2
a=84:2
a=42