15. (3 points) There are four people, A, B, C, and D. It is known that the average age of A, B, and C is 1 year older than the average age of the four people, the average age of A and B is 1 year older than the average age of A, B, and C, A is 4 years older than B, and D is 17 years old. How old is A $\qquad$?
【Solution】Solution: Let Jia's age be $x$ years, then Yi's age is $x-4$ years; the average age of Jia and Yi is: $(x+x-4) \div 2=x-2$ (years);

The average age of Jia, Yi, and Bing is: $x-2-1=x-3$ (years);
Bing's age is: $3(x-3)-2(x-2)=x-5$ (years);
$$
\begin{aligned}
x-3-1 & =(x+x-4+x-5+17) \div 4 \\
4 \times(x-4) & =3 x+8 \\
4 x-16 & =3 x+8 \\
x & =8+16 \\
x & =24 .
\end{aligned}
$$

Answer: Jia is 24 years old.
Therefore, the answer is: 24.