Question

a/b=12, b/c=20, what is ab + c ?

Answer 1

first eq. gives a=12b
second one: b = 20c
ab+c = 12bb+c = 12(20c)(20c)+c = ...
you can't expect to find a real number as answer, you would need one more equation. an expression in terms of one unknown (a , b, or c) is the best you can expect.