Minxia counted 40 green cars and 20 silver cars in the parking lot. if the number of green cars stays the same, how many more silver card would need to be added so the ratio of green cars to silver cars is 1 to 3?

What I did is that: I multiply 40 by 3 because 40 is 1/3 and I have to find the answer for 3/3 (Confusing, sorry I don't know how to explain it correctly). I got 120, subtract 20 from 120 beacause ther is 20 silver cards. The answer is 100