16. Party A excavates a canal, on the first day digging $\frac{1}{4}$ of the total length of the canal, on the second day digging $\frac{5}{21}$ of the remaining length, on the third day digging $\frac{1}{2}$ of the remaining length, and on the fourth day finishing the remaining 100 meters of the canal. Therefore, the length of the canal is $\qquad$ meters.
【Solution】Solution: Consider the total length of the canal as unit "1", then the fraction excavated on the first day is $\frac{1}{4}$, the fraction excavated on the second day is $\left(1-\frac{1}{4}\right) \times \frac{5}{21}=\frac{5}{28}$, and the fraction excavated on the third day is $\left(1-\frac{1}{4}-\frac{5}{28}\right) \times \frac{1}{2}=\frac{2}{7}$,
$$
\begin{array}{l}
100 \div\left(\left(1-\frac{1}{4}-\frac{5}{28}-\frac{2}{7}\right)\right. \\
=100 \div \frac{2}{7} \\
=350 \text { (meters) }
\end{array}
$$

Answer: The canal is 350 meters long.
Therefore, the answer is: 350.