11. If the ten-digit number $2016 \mathrm{ab} 2017$ is a multiple of 99, then $a+b=$

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11. If the ten-digit number 2016 $\overline{\mathrm{ab} 2017}$ is a multiple of 99, then $a+b=8$.
【Solution】Solution: According to the divisibility rule of 99, we know:
$$
\begin{array}{l}
\quad 20+16+\overline{a b}+20+17=99 . \\
\overline{\mathrm{ab}}=26 . \\
a+b=8 .
\end{array}
$$

Therefore, the answer is: 8.