Set the number the milestone gives you, pick which kind of sentence carries it, and pick the second fact. The first three cases all describe the mother of 39 and the son of 13 — two different milestone phrasings and two different second facts, one answer.
Elder was 26 at the younger’s birth; ratio 3 : 1 today
Mother 39 · son 13 · gap 26
Changing only the middle control rewrites the story and leaves the arithmetic untouched, which is the point of the model. To see a guard fire, ask for a gap of 26 with a total of 24 — two people 26 years apart cannot add up to 24.