A mother is 28 years older than her son. At some point in the future her age will be exactly three times his. How old will the son be then?

The 28-year gap never changes, so at that moment mother - son = 28 while mother = 3 × son. Substituting, 3s - s = 28, so 2s = 28 and the son is 14 (with the mother at 42). 42 is the mother's age then, not the son's. 7 comes from 4s = 28, which would be right if their ages ADDED to 28 rather than differing by it. 28 is the gap itself, which is never either person's age here.