Kevin is twice the age of his daughter Lucy and Lucy is three times as old as her daughter Millie. If their age is 120 then how old are Kevin, Lucy and Millie?

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