If it doesn't say which colour you add, there's more than one answer.
The scenario suggested above certainly works.
But there's also e.g.
Original ratio is 5:6
Final ratio is 1:2 which is equivalent to 5:10
Therefore 4 'parts' of yellow marbles have been added.
Therefore 1 'part' of yellow marbles = 7
Therefore you started off with 35 purple and 42 yellow, added 28 yellow to get 35 purple and 70 yellow, i.e. 105 altogether.
Start off with 20 purple and 24 yellow.
Add 4 purple and 24 yellow to give 24 purple and 48 yellow.
Think those are the only 3 possibilities. Was this a multiple choice question? If there was only one choice of answer that worked, I suppose that's ok
But otherwise you have to make assumptions, which isn't ok.