3. The father is now 35 years old, and the son is 7 years old. In how many years will the father be twice as old as the son?
Solution. Let's denote by $x$ the number of years it will take for the father to be twice as old as the son. After $x$ years, the father's age, $35+x$, will be twice the son's age, $7+x$, so we have

$$
35+x=2(7+x)
$$

from which we get $x=21$.

Therefore, the father will be twice as old as the son in 21 years.