11. (12 points) The three brothers, Big Mao, Second Mao, and Third Mao, Big Mao said to Third Mao: “When dad was 36 years old, my age was 4 times yours, and Second Mao's age was 3 times yours.” Second Mao said: “Yes, at that time, the sum of our ages was exactly half of dad's current age.” Third Mao said: “Now, the total age of the four of us is already 108 years old.” So, how old is Third Mao this year? $\qquad$
【Solution】Solution: Let's assume when Dad is 36 years old, Sanmao is $x$ years old, then Ermao is $3 x$ years old, and Damao is $4 x$ years old. Therefore, Dad's current age is: $2(x+3 x+4 x)=16 x$ (years),
$$
\begin{array}{r}
4 x+3 x+x+(16 x-36) \times 3+16 x=108 \\
72 x-108=108 \\
72 x=216 \\
x=3
\end{array}
$$

Then Sanmao is:
$$
\begin{aligned}
& (16 \times 3-36)+3 \\
= & 48-36+3 \\
= & 12+3 \\
= & 15 \text { (years) }
\end{aligned}
$$

Answer: Sanmao is 15 years old this year.
The answer is: 15.