Let's start with Frances' age. Let's say, the answer will come from Frances' age as his/her age was already given
Frances = 15
Based on your question, Judy is twice as old as Frances so...
15 (2) = 30 - Judy's age
And now, for Carmen's age (in terms of n)
30 - n = Carmen
Therefore, the answer to your question is 30 - n.