Maths problem

Oscar age should be 34, I dunno where the multiplication of 2 came from and it's connection to the problem, it came out of no where.

Because if we think about it, the mother is only 3 years older than Oscar and the father is 5 years older than the mother. So if 17 is really the answer then:

Oscar = 17
Mother = 17 + 3 = 20
Father = 20 + 5 = 25

Which is wrong since it was given in the problem that the father is 42.

While if:

Oscar = 34
Mother = 34 + 3 = 37
Father = 37 + 5 = 42

So yeah Oscar is 34 years old.

In my opinion , I think they mistake it thinking Oscar should have a young age, but it was not stated that "that" mother is Oscar parent. It just says "a" mother not "his".