The age difference between father and son is constant, which is 30 years old. 40- 10=30 (years old)

My son is 10 years old this year. When the son is 30 years old, the father is twice as old as the son. 30- 10=20

Twenty years later, the father is 40+20=60 (years old) and the son 10+20=30 (years old).

Twenty years later, my father is twice as old as my son.

60/30= twice the age difference between father and son (60-30=40- 10=30 years old)