5. (3 points) Wang Lei was born on National Day. If three times his age minus 8 is exactly the total number of days in the month he was born, then Wang Lei is $\qquad$ years old this year.
5. (3 points) Wang Lei was born on National Day. If three times his age minus 8 is exactly the total number of days in the month he was born, then Wang Lei is $\qquad$ years old this year.

【Analysis】Since National Day is in October, and October has 31 days, according to "three times his age minus 8 is exactly the total number of days in the month he was born," we know that three times Wang Lei's age minus 8 equals 31. Therefore, we first find three times Wang Lei's age, and then find Wang Lei's age.

【Solution】Solution: $(31+8) \div 3$
$$
\begin{array}{l}
=39 \div 3 \\
=13 \text { (years old); }
\end{array}
$$

Answer: Wang Lei is 13 years old this year.
Therefore, the answer is: 13.
【Comment】The key to solving this problem is to know that October has 31 days, and then use the quantitative relationship "three times Wang Lei's age minus 8 equals 31" to solve the problem.