6. (10 points) Given $1 ※ 2=1+2=3, 2 ※ 3=2+3+4=9, 5 ※ 4=5+6+7+8=26$, if $a ※ 15=165$, then $a=$ $\qquad$
【Solution】Solution: According to the operation rules, we have
165 is the sum of an arithmetic sequence, 15 is the number of terms in the arithmetic sequence, the common difference is $1, a$ is the first term,
$$
\begin{array}{l}
a=\left[165-\frac{15 \times(15-1) \times 1}{2}\right] \div 15 \\
=[165-105] \div 15 \\
=60 \div 15 \\
=4
\end{array}
$$

Therefore, the answer is: 4 .