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Present age of Father = f
Present age of Son = s
The age of the father is 5 times the age of his son.
After 18 years,
Age of Father = f+18
Age of Son = s+18
Then, the age of the father will be twice his sons age.
Substitute the value of f in (2) from (1)
Substitute the value of s in (1)
Present age of Father = f = 30 years old
Present age of Son = s = 6 years old