A)A:b=2=>a=2b
c=2
a-c=54=>a=c+54
________
a+b+c=?
a=2+54
a=56
b=56:2
b=28
56+28+2=86
B)a+b=83
a-b=15=>a=b+15
_________
a, b=?
b+15+b=83
2b+15=83
2b=83-15
2b=68
b=68:2
b=34
a=34+15
a=49
C) a=3b
a-b=56=>a=56+b
_______
a,b=?
3b-b=56
2b=56
b=56:2
b=28
a=28×3
a=84