Let's say x to the daughter's age, since the father's age is four times the daughter's age, his age will be 4x. 18 years later, the daughter's age will be x+18 and the father's will be 4x+18. And we know that 18 years later the father's age will be twice the daughter' so , 4x+18 = 2(x+18) and 4x+18=2x+36 then, 4x-2x=36-18,
2x=18 then divide both sides by 2, x = 9. The daughter's age was x, so she is 9 years old.