Let millie's age be "X" then lucy's age would be "3X" then kevin's age is 2 x (3X) = "6X" if the sum of their ages is 120 that means X + 3X + 6X = 120 10X = 120 so X = 120/10 = 12 by substituting, millie is 12 years old lucy is 3x12 = 36 years old and kevin is 6x12 = 72 years old