【Question 10】
7 years ago, father's age was 5 times the daughter's age, this year father's age is 3 times the daughter's age, then in $\qquad$ years, father's age will be 2 times the daughter's age.
【Analysis and Solution】
Age problem.
Let the daughter's age this year be $x$ years old, then the father's age this year is $3 x$ years old; according to the problem, we get $5(x-7)=3 x-5$;

Solving for $x$ yields $x=14$;
This year, the daughter is 14 years old, and the father is $14 \times 3=42$ years old; the age difference between the father and the daughter is $42-14=28$ years;

When the father's age is twice the daughter's age, the daughter is $28 \div(2-1)=28$ years old; it will be $28-14=14$ years later when the father's age is twice the daughter's age.