the system of linear equations below has a unique solution for all but one value of a :

Question

5 x - 5 y = 25
48 x + a y = 240

What is this exceptional value for a ?

Answer 1

If the two lines would be parallel, we could not find a solution,
that happens when

5/48 = -5/a
5a = -5(48)
a = -48