The sum of the present ages of a father and his son is 99 years. When the father was as old as his son is now, his age was four times the age of the son at that time. The ratio of the present ages of the son and the father is -

Let present ages be: father = F, son = S. Given: F + S = 99. Age difference = F - S (constant). When father was S years old (F - S years ago), son was S - (F - S) = 2S - F years old. At that time, father's age (S) was 4 times son's age: S = 4(2S - F). Solving: S = 8S - 4F → 4F = 7S → F:S = 7:4. Converting to son:father ratio gives 4:7.