[Number Theory]
Problem: Let $N=888,888 \times 9,999,999$. Compute the sum of the digits of $N$.

Solution: Write $N$ as

$$
\begin{aligned}
& (10,000,000-1) \cdot 888,888 \\
= & 8,888,880,000,000-888,888 \\
= & 8,888,879,111,112 .
\end{aligned}
$$

The sum of the digits of $N$ is 63 .

Final answer: 63