5. (8 points) It is defined that $1 ※ 2=0.1+0.2=0.3, 2 ※ 3=0.2+0.3+0.4=0.9, 5 ※ 4=0.5+0.6+0.7+0.8=2.6$. If $a ※ 15=16.5$, then $a$ equals $\qquad$ .
5. (8 points) Given $1 ※ 2=0.1+0.2=0.3, 2 ※ 3=0.2+0.3+0.4=0.9, 5 ※ 4=0.5+0.6+0.7+0.8=2.6$, if $a ※ 15=16.5$, then $a$ equals 4.
【Solution】Solution: According to the operation rule,
16.5 is the sum of an arithmetic sequence, 15 is the number of terms in the arithmetic sequence, the common difference is $0.1, a$ is 10 times the first term,
$$
\begin{array}{l}
a=\left[16.5-\frac{15 \times(15-1) \times 0.1}{2}\right] \div 15 \times 10 \\
=[16.5-10.5] \div 15 \times 10 \\
=6 \div 15 \times 10 \\
=4
\end{array}
$$

Therefore, the answer is: 4.