31. When Vitya becomes twice as old, Kolya will be 4 years younger than Vera. Last year, Kolya was half as old as Vera, and 3.5 times younger than Vitya. How old is each of them now?
31. Let Kolya be $x$ years old last year, then Vera is $2 x$ and Vitya is $3.5 x$. Now Kolya is $(x+1)$ years old, Vera is $(2 x+1)$ years old, and Vitya is $(3.5 x+1)$ years old. Vitya will be twice as old in $(3.5 x+1)$ years, then Kolya will be $(4.5 x+2)$ years old $((x+1)+$ $+(3.5 x+1)=4.5 x+2)$, and Vera will be $(5.5 x+2)$ years old $((2 x+1)+(3.5 x+1)=5.5 x+2)$. According to the condition, we have

$$
(5.5 x+2)-(4.5 x+2)=4
$$

from which $x=4$. Therefore, Kolya is currently 5 years old, Vera is 9 years old, and Vitya is 15 years old.